I don't think the term in question has a broad, established legal meaning. From I've seen, it's been referred to in a number of cases in US courts. So, I don't suppose we have a clear equivalent for it in Persian. I suggest "همردیف عملی" because the term is used for defining the status and functional/practical role of some parties in relation to the status and functional/practical role of a close family member:

Instances (probably from your own text):

To illustrate what it meant by "functional equivalent," the Bettis court cited a case from this District. Sullivan v. Ford Motor Co., No. 97-CV-0593 (RCC), 2000 WL 343777 (S.D.N.Y. Mar. 31, 2000). In Sullivan, the court awarded solatium damages to an aunt who was the legal custodian of her nephew, "performed all of the duties and responsibilities of a natural or adoptive parent," and was "the only family with whom [the nephew] shared his life on a daily basis from the time he was an infant of 12 months until his death." Id. at *11 & n.3. This functional and legal responsibility allowed for such an award notwithstanding New York State's stringent definition of "immediate family member" for the purpose of negligent infliction of emotional distress claims.

Further FSIA terrorism cases have elucidated and refined the "functional equivalent" language from Bettis. In Estate of Heiser v. Islamic Republic of Iran, the court emphasized the importance of cohabitation, noting that "in those rare cases in which the parties at issue had lived in the victim's immediate household and had been in other important respects like a spouse, parent, sibling, or child to the victim, circumstances may require a slight stretching of the immediate-family requirement . . . ." 659 F. Supp. 2d 20, 29 (D.D.C. 2009). On this logic, the Heiser court awarded solatium damages to two non-adoptive stepfathers who were found to be the "functional equivalents of fathers" because they lived in the same household while their stepsons were minors and treated them as their own sons in every sense (financially, emotionally and socially).

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