What is the sister's legal position, and is she sinful if she neglects the child of unknown parentage whom she sponsored, and his condition deteriorates due to her negligence? Is it permissible to call her "Umm so-and-so" (the child's name)? And is the child's living with her alone permissible in Islamic law?

Sponsoring an orphan and a child of unknown parentage are acts of righteousness. However, the forbidden adoption (تبني) is to attribute the child to other than his father. Neglecting the sponsored girl and squandering her rights is impermissible. The guardian must return her to the responsible institution; otherwise, she will be sinful. It is not permissible to call the female guardian "Umm Fulan" (mother of so-and-so) because it is an acknowledgment of a sin. There is no objection to the child being alone with her as long as he has not reached puberty. However, once he approaches it, he should be prevented from being alone with her and treated as a stranger, unless she has breastfed him to the extent that it creates a Mahram relationship.