What is the ruling on the husband's mother interfering in her son's marital life, and how can the wife deal with her given the husband's refusal to intervene?

If the mother-in-law's actions cause distress, the wife should be patient with her and help her husband in showing her filial piety. There is no harm in gently suggesting to him that he advise her. It is part of a woman's good character to associate with her in-laws, in order to gain her husband's affection. She can visit her and inquire about her, seeking reward [from Allah], even if it is not legally obligatory, even if the husband requests it, for a woman's obedience to her husband is in matters of marriage and its related affairs. It must be noted that the default assumption for a Muslim is innocence, so it is not permissible to accuse them of anything shameful without clear evidence.