Tanya Katrina Williams, now known as Katrina Wimer, appeals the district court’s order changing the residential custody of her son, Dalton. Following the order, Dalton’s residential custody was placed with his father, Patrick Kenneth Williams, while residential custody of Dalton’s full sister, Ashley, remained with Katrina. On appeal, Katrina contends that the separation of siblings effected by the district court’s order violated the legislative mandate in K.S.A. 2003 Supp. 60-1610(a)(5)(B), which permits divided custody only in “exceptional cases.” Finding that the district court was presented with an exceptional case, we affirm.
*843Following their 1996 divorce, Katrina and Patrick had joint custody of Ashley (d.o.b. 06/30/89) and Dalton (d.o.b. 08/01/94), with Katrina as the primary residential custodian. In February 1997, Katrina and the children moved to Oklahoma. Later, she married Troy Wimer, and they have a son, Robert. Patrick remained in Kansas, married Lori, and had a son, Wyatt. Lori also has a son, Tyson, from a prior relationship.
During spring break visitation in March 2003, Patrick took Dalton to see Steven A. Cagle, LSCSW, with Family Practice Associates in Dodge City, to address some concerns Patrick had with Dalton’s behavior and to assess the sincerity of Dalton’s expressed desire to live with his father. Cagle had three sessions with Dalton prior to the conclusion of spring break visitation. In these sessions, Cagle observed that Dalton appeared lethargic and unhappy; Dalton said he did not have a close relationship with his older sister and younger half-brother and insisted that he wanted to live with his father. In the last session, Cagle specifically questioned Dalton about a fire he attempted to set next to his mother’s residence. Dalton said his motive in setting the fire was that he did not want to live at his mother’s house anymore. Cagle was concerned that Dalton appeared to lack remorse; Cagle characterized Dalton’s actions as a “ciy for help.”
In April 2003, Patrick filed a motion to change Dalton’s residential custody from Katrina to himself. Acknowledging that Ashley was apparently doing well in her mother’s custody, Patrick did not ask to change his daughter’s residential custody. In his motion, Patrick alleged, inter alia, that Dalton was exhibiting signs of. behavioral and emotional problems; that Dalton had attempted to set fire to Katrina’s house and Katrina had not dealt with the situation appropriately; that Dalton had expressed a strong desire to live with his father; and that Patrick was better suited to meet Dalton’s current needs.
During the pendency of the custody motion, Patrick asked the court to define his summer visitation because Katrina had allegedly said she intended to deny Patrick any summer parenting time. Based on the parties’ agreement, the court ordered that Ashley and *844Dalton would visit Patrick from May 24 to June 8, and from June 21 to July 26, 2003.
In May 2003, Katrina moved to transfer the case to Oklahoma pursuant to the Uniform Child-Custody Jurisdiction and Enforcement Act, K.S.A. 38-1336 et seq. The Oklahoma court declined to exercise jurisdiction until the Kansas court relinquished jurisdiction. Finding the Kansas situs to be more convenient, the Kansas court retained jurisdiction.
On July 28, 2003, Cheryl Legg of Southeastern Psychiatric Services in McAlester, Oklahoma, met with Katrina and Dalton for an hour. On July 30, Ms. Legg met with just Dalton for an hour and, later in the day, met with Dalton and Ashley. On July 31, Ms. Legg had a family therapy session with Wimer, Katrina, Ashley, and Dalton. Ms. Legg testified that the intensive counseling sessions over a 3-day period were necessary to prepare for the upcoming court hearing. Ms. Legg also characterized Dalton’s fire-setting as a cry for help, although she opined that the relief he sought was from being placed in the middle of a tug-of-war between his mother and father. Ms. Legg opined that Dalton and Ashley had a strong bond and that they should not be separated.
On August 6, 2003, the court held an evidentiary hearing on the custody motion. Testimony was taken from the two therapists, Cagle and Legg, as well as from Patrick, his wife Lori, and Katrina. The parties stipulated to the proffered testimony of Katrina’s husband, Troy. At the conclusion of the hearing, the district court announced from the bench that it was granting the motion to change Dalton’s residential custody to Patrick.
In making its decision, the district court acknowledged that both parents love their children and that there is a presumption requiring the court to keep siblings together unless there is an exceptional case. The court noted that it was considering the history of the case, including Katrina’s lack of cooperation. The court expressed some frustration with Katrina’s attempt to transfer jurisdiction to Oklahoma after the commencement of the custody litigation. The court indicated a belief that Katrina obtained the 11th hour intensive counseling with Ms. Legg to assist in resisting a custody *845change, rather than to timely address Dalton s problems. Further, the district court specifically found:
“8. That the truth is that there has been friction in the relationship between Dalton and Ashley, even though they love one another.
“9. That there has been an expressed preference by Dalton, in a neutral setting, when there was not a motion pending, that he stay with his father, and there were rational reasons.
“10. That those rational reasons were:
a. That he viewed his mother’s parenting role as geared toward his older sister.
b. That he was discriminated against in a way that caused him to be considered less in the family relationship than his sister was.
c. That he had a preference to stay with his father, who apparently had the ability to show attention when necessary, and also participate in activities that Dalton apparently identified with and benefited from.
“11. That the physical structures of the homes favor the Respondent’s arrangement and the space available for Dalton is more favorable opposed to Petitioner’s home.
“12. That the motivation to maintain a long term view of the parenting roles for both parents would be promoted by allowing Dalton to live with his father.
“13. That there is no persuasive evidence that the sibling relationship will be damaged.
"14. That there has been a lack of communication between the parties that can be improved, if tire children are living in each home.
“15. That there is evidence that is persuasive that there has been a tug-of-war existing and that Dalton feels he is in the middle of it, and that it exists because there is a conflict regarding where he’s going to reside. It can be resolved by changing his residence.
“16. That it would be in Dalton’s best interests if he were to reside with his father.
“17. That all of the facts together, from the beginning of this case, do constitute a material change of circumstances to make that change of custody for Dalton.”
The court made additional findings that are not germane to this opinion. In response to a motion to alter or amend, the district court acknowledged that it had not specified the findings which made this an exceptional case. It then found that, pursuant to 60-1610(a)(5)(B), this is an exceptional case justifying a divided custody order based generally on the totality of the circumstances and specifically on Dalton’s stated preference to live with his father and Dalton’s stated reasons for that preference.
*846Katrina states the question on appeal to be: “Were sufficient facts established for the district court to find that an exceptional case existed, as a matter of law, to justify the order to divide custody of a brother and sister between the parents?” Appellant’s brief begins by challenging a few of the district court’s findings as being either unsupported by the evidence or controverted by other evidence. Then, the argument transforms into a discussion of why Katrina believes the parties’ difficulties are “ordinary, common and frequent” for similarly situated split families and why Katrina believes the legislature restricted divided custody to protect the bond between siblings. The conclusion she reached is that this case presents no exceptional circumstances which would justify divided custody, as a matter of law.
We perceive appellant’s blended argument can be better analyzed by separating it into two questions: (1) whether the district court’s findings were supported by substantial competent evidence; and (2) whether the district court’s findings were sufficient to support the legal conclusion that this is an exceptional case. Ordinarily, appellate review of a district court custody order is for an abuse of discretion. See In re Marriage of Whipp, 265 Kan. 500, 502, 962 P.2d 1058 (1998). Here, however, we are called upon to interpret and apply K.S.A. 2003 Supp. 60-1610(a)(5)(B), malting the following standard more appropriate:
“Where the trial court has made findings of fact and conclusions of law, the function of an appellate court is to determine whether the findings are supported by substantial competent evidence and whether the findings are sufficient to support the trial court’s conclusions of law. Substantial evidence is evidence which possesses both relevance and substance and which furnishes a substantial basis of fact from which the issues can reasonably be resolved. [Citation omitted.]” Unrau v. Kidron Bethel Retirement Services, Inc., 271 Kan. 743, 747, 27 P.3d 1 (2001).
SUBSTANTIAL COMPETENT EVIDENCE
The judge hearing the custody motion had previously presided over the parties’ disputes. Therefore, we should afford the judge due deference in his consideration of the history of the case and in his characterization of the parties’ motives.
Katrina stipulated to the qualification of Steven Cagle as an expert. Cagle testified that Dalton sincerely wanted to live with his *847father. He further described Dalton’s reasons for wanting to move from his mother’s residence, which included problems in the relationship between Dalton and Ashley. Granted, Katrina’s expert, Cheryl Legg, provided conflicting testimony on these points. However, we do not function as a trier of fact and must defer to the district court’s determination of which testimony is more persuasive. If we refrain from reweighing the evidence and assessing witness credibility, then we must uphold the factual findings that deal with Dalton’s living arrangement preference and with Dalton’s relationship with his sister.
The district court’s finding that Katrina engaged the services of a therapist to prepare for court, rather than to timely deal with Dalton’s needs is supported by evidence. The attempted fire-setting incident occurred prior to Dalton’s March visitation with his father. Katrina did not effect Dalton’s first intake with Cheryl Legg until July 28, some 9 days prior to the custody hearing, and after a mediator had suggested she obtain counseling for Dalton. Even Ms. Legg characterized the fire-setting act as a cry for help. Further, Ms. Legg opined that Dalton was suffering from adjustment disorder and that he displayed symptoms of depression. She said that she personally handled the case because the Katrina family unit would require intensive counseling. However, she admitted the primary reason for conducting four sessions in 3 days was to prepare for the upcoming court case, and she intimated that Katrina should have sought out her services earlier. From the evidence, one could reasonably infer that Katrina ignored clear signs that Dalton was a disturbed child until she was motivated to obtain evidence for the custody hearing.
The evidence that Dalton would have his own room at Patrick’s house, as opposed to sharing a room at Katrina’s house, was uncontroverted and supported the court’s finding. The evidence supported that Patrick was more attuned and attentive to Dalton’s interests, as opposed to Katrina’s efforts to get Dalton involved with Ashley’s projects.
However, the findings which suggest that separating Dalton and Ashley would result in better communication and cooperation between the parents do not appear to have any factual support in the *848record. Neither therapist proffered that opinion and, therefore, the judge’s findings in that regard should be disregarded.
In summary, the evidence was sufficient to support the district court’s findings that Dalton wanted to live with his father; that Dalton perceived his mother gave preferential treatment to Ashley; that the relationship between Dalton and Ashley was strained; that Dalton was conflicted by the custody fight; that Katrina had failed to timely recognize and address Dalton’s manifestations of emotional or behavioral problems; that Patrick’s residence would provide Dalton a room of his own; and that Patrick would better accommodate Dalton’s interests. However, all those findings are penultimate. The primary criterion in a custody determination is the best interests of tire child. See LaGrone v. LaGrone, 238 Kan. 630, 632-33, 713 P.2d 474 (1986). The district court’s collateral findings were supported by tire evidence, and tiróse findings supported the determination that divided custody was in the best interests of Dalton.
EXCEPTIONAL CASE
K.S.A. 2003 Supp. 60-1610(a)(5) provides the types of residential arrangements a court may consider in a divorce case, listed in order of preference. Subsection (B) provides: “In an exceptional case, the court may order a residential arrangement in which one or more children reside with each parent and have parenting time with the other.” K.S.A. 2003 Supp. 60-1610(a)(5)(B). Thus, arguably, the district court does not have tire discretion to divide the custody of children of the same parents where there are no exceptional circumstances. See LaGrone, 238 Kan. at 635 (Lockett, J., concurring in part, dissenting in part). Interestingly, neither party mentions die fact that under any arrangement, both Dalton and Ashley will, by necessity, be separated from a half-brother.
Our next step, then, is to determine whether the findings which are supported by substantial competent evidence are sufficient to conclude that the district court was presented with an “exceptional case.” LaGrone is the only case on divided custody cited by either party.
*849The LaGrone majority opinion states the obvious: “The statute does not define what an exceptional case is, nor does it give examples.” 238 Kan. at 633. However, the opinion does not attempt to provide a definition, but rather simply states that “[t]he facts before the trial court in this case were unusual.” 238 Kan. at 633. Apparently, the unusual character of that situation was that the father had, at times, been the primary caretaker of the child placed in his custody. The dissent noted the testimony that “the two children loved each other, played together and got along well,” and that both parents were fit, both loved their children, and both took good care of them. 238 Kan. at 634-35. The dissent could not find any exceptional circumstances, rejected the majority’s reliance on perpetuating the status quo, and lectured that “[fjamily ties between children of the same parents should not be treated lightly.” 238 Kan. at 635.
No reasonable person can seriously doubt the wisdom of Justice Lockett’s admonition regarding sibling relationships. See Henderson v. Henderson, 537 So. 2d 125, 128 (Fla. Dist. App. 1988) (family unit should not be further fractured by a divorce). Children should not be deprived of a sibling relationship simply to accommodate the wants and needs of their separated parents. However, when the children’s own welfare is implicated, the nonseparation rule is subordinate to the best interests of the child rule. Otherwise, we might well be employing a rule developed for the benefit of the children of divorcing parents to actually defeat that which would be beneficial to the child.
Perhaps it is best that neither statutory law nor case precedent provides a definition for “exceptional case.” The determination is too important to be subjected to a mechanical application of an artificial litmus test containing three factors or two prongs. However, we feel comfortable definitively stating that, when the district court makes a finding, supported by substantial competent evidence, that divided custody is in a child’s best interests, the court has met the requirement of establishing an “exceptional case.”
Here, the district court found divided custody to be in Dalton’s best interests. That finding was supported by substantial competent evidence. As a matter of law, this was an exceptional case.
*850Affirmed.