Is the husband considered undutiful to his mother if he asks her to live with his siblings, knowing that she lived with him for eight months after he and his wife agreed to live separately, and her living with them caused problems between them, and she insists that he send his zakat money to his non-needy siblings?

It has been previously clarified that the wife has the right to an independent dwelling. However, if you are not harmed by the presence of your husband's mother, then you do not have the right to demand her expulsion if that would expose her to destitution. As for the mother's insistence on her son to pay zakat (definition) to his siblings: if the siblings are not among those eligible for zakat or if their maintenance is legally obligatory upon him, then it is not permissible for him to obey her. However, if they are eligible for zakat and their maintenance is not obligatory upon him, then paying zakat to them is more preferable. We advise the questioner to fear Allah and to be a helper to her husband in honoring his mother, and to treat her as she would treat her own mother.