Such factors are by no means determinative of the ultimate issue of the best interests of the children, but are to be given due consideration in determining whether there should be a change in custodial arrangements. Indeed, "[t]he courts should be reluctant to transfer custody of young children who have been with their mother since birth" ( Aberbach v Aberbach, 33 N.Y.2d 592, 593) and such "custody should be continued unless it is demonstrated that the custodial parent is unfit or perhaps less fit" ( Martin v Martin, 74 A.D.2d 419, 426). Not only does it appear that the trial court failed to give due consideration to these factors in determining to order a change in custody, the proof before the court failed to demonstrate the "unfitness" of the wife, in any sense, or that she was "less fit" ( Aberbach v Aberbach, 33 N.Y.2d 592, supra).

Extraordinary circumstances are not a prerequisite to obtaining a modification; rather, the "standard ultimately to be applied remains the best interests of the child when all of the applicable factors are considered" ( Friederwitzer v. Friederwitzer, 55 NY2d 89, 95; see Matter of Krom v. Comerford, 57 NY2d 704, 705). Relevant considerations include whether the alleged change implicates the "fitness" of one of the parties ( see Aberbach v. Aberbach, 33 NY2d 592, 593), the nature and quality of the relationships between the child and the parties ( see Matter of Emanuel S., 78 NY2d at 182; Matter of Tropea v. Tropea, 87 NY2d 727, 740-741) and the existence of a prior agreement ( see Eschbach v. Eschbach, 56 NY2d 167, 171; Friederwitzer, 55 NY2d at 94-95).

This priority is afforded the first determination of custody in the belief the stability this policy will assure in the child's life is in the child's best interests. ( Friederwitzer v Friederwitzer, supra, at p 94; Corradino v Corradino, 48 N.Y.2d 894; Matter of Nehra v Uhlar, supra; Obey v Degling, supra; Dintruff v McGreevy, 34 N.Y.2d 887; Aberbach v Aberbach, 33 N.Y.2d 592; People ex rel. Selbert v Selbert, 60 A.D.2d 692.) But as this court noted in Friederwitzer, "[n]o agreement of the parties can bind the court to a disposition other than that which a weighing of all the factors involved shows to be in the child's best interests ( People ex rel. Wasserberger v Wasserberger, 42 A.D.2d 93, 95, affd on opn below 34 N.Y.2d 660)."

The State's special concern with the interrelated, although of course not identical, matters of custody and visitation is not newborn. While contemporary courts most often will defer to the terms negotiated by the parents (Matter of Ebert v Ebert, 38 N.Y.2d 700, 703, supra; Aberbach v Aberbach, 33 N.Y.2d 592; Pilpel Zavin, Separation Agreements: Their Function and Future, 18 L Contemp Prob 33, 34-35, n 2), for a long time such agreements were not enforceable (2 Foster — Freed, Law and the Family, § 29.21, p 535). Today, however, when, as in this case, courts are called upon to interpret or apply an agreement on this subject, in this State they must not hestiate to resolve the issue (Domestic Relations Law, § 240).

This is all the more so here where, other than the children's wishes, there was no change of circumstances, especially with respect to the crucial matter of fitness, to justify a modification of the custodial arrangements which the parties themselves had determined was best for these young children. (Aberbach v Aberbach, 33 N.Y.2d 592; Matter of Feldman v Feldman, 45 A.D.2d 320, 324.) Since there was no material change of circumstances, there was no foundation for this petition.

His sister, about four and a half years younger, has lived with the mother without interruption, the father never having made efforts to secure her custody. Courts should be reluctant to permit separate custody of siblings (Aberbach v Aberbach, 33 N.Y.2d 592; People ex rel. Borella v Borella, 21 A.D.2d 871; Matter of Lang v Lang, 9 A.D.2d 401, 405, affd 7 N.Y.2d 1029, supra; 2 Foster-Freed, Law and the Family, § 29.13). Young brothers and sisters need each other's strengths and association in their everyday and often common experiences, and to separate them, unnecessarily, is likely to be traumatic and harmful.

Kristin Booth Glen for respondent. Judgment affirmed, without costs (Aberbach v Aberbach, 33 N.Y.2d 592; Matter of Berlin v Berlin, 21 N.Y.2d 371; see People ex rel. Wasserberger v Wasserberger, 34 N.Y.2d 660, 661). Concur: Chief Judge BREITEL and Judges JASEN, GABRIELLI, JONES, WACHTLER, FUCHSBERG and COOKE.

The mother's contention that the Family Court erred in considering the findings of the court-ordered reports is unpreserved for appellate review ( see Matter of Diaz v. Santiago, 8 AD3d 562, 563). In any event, the Family Court, with the consent of the parties, properly considered the reports, which recommended that custody of the child be given to her father ( see Aberbach v. Aberbach, 33 NY2d 592, 593). Moreover, the court assessed the credibility of the parties in favor of the father and determined that an award of sole custody to the father was in the best interests of the child.

The record supports the conclusion that the father failed to establish a change in circumstances warranting a change in residential custody. Nor did the father demonstrate that the mother was less fit than he as a parent ( see Aberbach v. Aberbach, 33 NY2d 592; Matter of McNerney v. McNerney, 242 AD2d 385; cf. Matter of Faunteleroy v. Mercado, 5 AD3d 482). Accordingly, the Supreme Court properly continued residential custody of Brandon with his mother. The father's remaining contention is unpreserved for appellate review.

Turning to the determination that petitioner's visitation should be supervised, we note that "[w]hether visitation is appropriate is a matter left to Family Court's sound discretion * * * and its findings, to which deference is to be accorded, will not be disturbed on appeal unless they lack a sound basis in the record" (Matter of Shawn Y. [David Y.], 263 A.D.2d 687, 688 [citations omitted]). Upon our consideration of the testimonial evidence and the reports submitted pursuant to court order without objection and after affording the parties an opportunity to rebut the conclusions reached therein (see, Aberbach v. Aberbach, 33 N.Y.2d 592, 593; Matter of Thaxton v. Morro, 222 A.D.2d 955, 957), we find sufficient evidence to support a temporary restriction on visitation. Family Court properly grounded its determination upon,inter alia, petitioner's disparagement of respondent to the children (see, Matter of Belden v. Keyser, 206 A.D.2d 610) and the effect that petitioner's psychological difficulties, including clinical depression and uncontrollable anger, had upon the exercise of his judgment when communicating or interacting with the children (see, Matter of Thaxton v. Morro, supra; cf., Matter of Stewart v. Stewart, 222 A.D.2d 895). While finding that supervised therapeutic visitation is wholly appropriate, we must agree with petitioner that Family Court improperly delegated its authority to a counselor to determine the best interests of the children in the structure of supervised visitation and thereafter (see, Matter of Millett v. Millett, 270 A.D.2d 520, 703 N.Y.S.2d 596, 598; Matter of Sullivan County Dept. of Social