Is it permissible to host my mother's maternal uncle's wife, the sorceress, and her children, and to visit them and eat at their place? And is she considered a disbeliever, such that it is impermissible to return her greeting of peace?

If a woman deals with sorcerers and diviners, it is an evil, and the prohibition against approaching them is more severe than merely believing them. If she herself is a sorceress, the matter is even more grave, and some scholars have deemed a sorcerer to be a disbeliever. However, it is not permissible to declare her an individual disbeliever except under specific conditions, and the default is that she is a Muslim, so one should greet her with salam and return her greeting. If you are safe from her harm, there is no objection to visiting her and advising her. But if you fear her harm, then boycotting her is legislated. Boycotting for more than three days is permissible if speaking to her brings about a deficiency in religion, or harm to oneself, or to worldly affairs. There is no objection to eating from her food unless it is proven to be from purely unlawful wealth; it is disliked if the wealth is mixed.