Is it permissible for one harmed by his sister’s continuous abuse and severing of family ties to boycott her and her family members who support her in her abuse?

It is not permissible to sever ties of kinship, such as with a sister and household members. Rather, one must endure their harm and continue to maintain ties with them in obedience to Allah and His Messenger. The one who maintains ties is not the one who merely reciprocates. Maintaining ties has degrees, the lowest of which is abandoning estrangement and maintaining ties with them through speech or greetings. Maintaining ties of kinship can be through money, assistance, warding off harm, a cheerful countenance, and supplication. The comprehensive meaning is: delivering as much good as possible and warding off as much evil as possible according to one's capacity. If they are disbelievers or transgressors, then boycotting them for the sake of Allah is a form of maintaining ties with them, while striving to admonish them and supplicate for them in their absence.