Is the son's possession of his mother's house 100% legitimate, especially since she transferred its ownership to him after his brothers assaulted her and seized the apartments, bearing in mind that he is not asking about the legal aspect?

It is not permissible for parents to favor some children over others in giving gifts unless they give the remaining children something else in return. The prohibition becomes stricter if the intention is to deprive heirs of their right. If the gift is completed with its conditions and the donee takes possession of it, then it is his property. However, if it is merely a written document stipulating ownership after death, then it is a will for an heir, and it is not valid unless approved by the adult heirs. The brothers and their wives must fear Allah and show kindness to their mother.