Is my reluctance to stay long at my mother's house or eat there, due to the presence of a maid who flirts with men, communicates with Saudi youth, and threatens with witchcraft, and my mother doesn't believe me, considered an impermissible act?

Maintaining ties with one's mother is confirmed in Islamic law, even if she is a disbeliever, based on the Almighty's saying: "But if they endeavor to make you associate with Me that of which you have no knowledge, do not obey them, but accompany them in [this] world with appropriate kindness." [Quran 31:15] And based on the Prophet's (peace be upon him) saying to Asma: "Maintain ties with your mother." This maintaining of ties should be according to one's capacity. If long visits are feared to cause harm, then they should be short. Special food can be brought when visiting her if her food is feared to cause harm. As for the daughters visiting their grandmother, it should be in your company or their mother's company. If the maid's corruption is proven, try to persuade your mother to dismiss her; otherwise, threaten to report her to the responsible authorities.