What is the husband's share of his mother's inheritance, how can he preserve it, and what should he do in light of his mother's and sisters' actions?

A mother must be fair among her children in gifting, and it is forbidden for her to favor one of them without a legitimate reason, due to the saying of the Prophet, peace and blessings be upon him: "Fear Allah and be just among your children." If there is no legitimate reason, then the gift must be returned or adjusted so that each child receives the same as the other. The Prophet, peace and blessings be upon him, commanded the return of the favoritism, saying: "Do not make me a witness, for I do not witness injustice," and in another narration: "So return it." This must be returned during the lifetime of the giver, and if he dies, it is returned after his death. As for a will to an heir, it is not valid unless approved by the remaining adult, sane heirs.