What is the ruling on a son interacting with his wife's sister's daughter, who was raised in their home but was not nursed by the son's mother?

There is no objection to raising the wife's sister's daughter, provided that her correct lineage and the laws of inheritance are preserved, and that she is considered a non-mahram to the foster parent's sons upon reaching puberty, so it is not permissible to be in seclusion with her or to look at her. It is better for the girl to remain with her parents, or to be returned to them after her infancy.