All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
We have previously underlined the enjoined dutifulness towards parents even if they are non-Muslim; please refer to fatwa 85180. The obligation to observe dutifulness and kindness towards the mother is even more emphasized; it was reported on the authority of Abu Hurayrah that a man came to the Prophet, sallallaahu ʻalayhi wa sallam, and said, "O Messenger of Allaah, tell me, which of the people has most right to my good companionship?" He, sallallaahu ʻalayhi wa sallam, said, "Your mother." He said, "Then who?" He said, "Then your mother." He said, "Then who?" He said, "Then your mother." He said, "Then who?" He said, "Then your father." [Al-Bukhari and Muslim]
Your father has no right to prevent you from maintaining ties of kinship with your mother. However, we advise you to be kind and gentle towards your father and wisely try to convince him that what he is commanding you to do with your mother constitutes severing the ties of kinship, which is prohibited. You may seek the help of his relatives or close friends whom you believe will be able to convince him. You are also advised to implore Allaah to help you convince him. If he is convinced, all praise be to Allaah; otherwise, you should maintain ties of kinship with your mother in a manner that does not make your father angry. If you cannot visit her, then you can opt for other means of communication.
Lastly, we would like to point out that the greatest manifestation of dutifulness towards your mother is seeking her repentance and guidance by means of advice to revert back to Islam; you may seek the help of another person to advise her in this regard. You should implore Allaah to bless her with guidance. We ask Allaah to guide her back to the Truth graciously. For more benefit, please refer to fatwa 87672.
Allaah knows best.