Is my brother's illness psychological, or is what afflicted him sorcery or the evil eye, and what should we do with my aunt after accusing her of performing sorcery on my brother?

It is not possible to determine whether your brother is afflicted by magic or a psychological illness based solely on a question. It is not permissible to accuse your aunt of witchcraft without clear proof. Even if it were proven, it is not advisable to sever ties with her. Instead, she should be advised, commanded to do good, and forbidden from evil, while continuing to honor and maintain kinship ties with her, in accordance with the Hadith (definition): "O 'Uqbah, maintain ties with those who cut you off, give to those who deprive you, and overlook those who wrong you." If your mother orders you to cut ties with her, explain the Islamic ruling to her and admonish her. If she insists, do not obey her in severing kinship ties, and maintain ties with your aunt without your mother's knowledge, unless the accusation of your aunt practicing witchcraft is proven with acceptable evidence.