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 is His slave and Messenger.
We ask Allaah to grant you and your brothers more steadfastness, to cure your mother and make her the comfort to your eyes. We hope that you have evidence for accusing your father of bewitching your mother because accusing someone without having evidence is a serious matter, especially in the case of accusing the father.
However, even if it is proved that your father has bewitched your mother; it does not drop his right of you being kind to him and having good relations with him. Therefore, you should keep ties with him to an extent that does not cause harm to you. If visiting him is not possible (due to the harm he may afflict you with), then you may stay in contact with him by phone and the like. This may help you gain his love, enable you to advise him and guide him. You must try to do so, because being kind to him is one of the greatest acts of worship. For more benefit, please see Fataawa 87019, 86982 and 84793.
Finally, you should try to seek treatment for your mother. If you strongly believe that the reason of her illness is magic then you may perform Ruqyah for her. For more details, please refer to Fataawa 81667 and 81643.
Allaah Knows best.