Is it permissible for the mother to dictate to her son's wife whom she communicates with, and is obedience to her obligatory in this regard, or does the matter concern the husband alone? And does the mother have guardianship over her son's wife?

Your mother has no right to interfere in your wife's affairs or prevent her from visiting your relatives, because this does not concern her. You should advise her gently, for "from the excellence of a person's Islam is his leaving what does not concern him." Maintaining kinship ties is obligatory, and your mother is not permitted to prevent you from visiting your maternal aunts without a legitimate Sharia (definition) reason. You are not allowed to obey her in this, for "obedience is only in what is right." It is wise for you to visit your maternal aunts accompanied by your wife without your mother's knowledge, in order to avoid her anger. Know that among the most important objectives of Islamic law is for Muslims to be in harmony and affection, and that reconciling disputes and striving for it is among the best acts of devotion.