Is a woman's boycott of her husband's family, who wronged her, accused her of witchcraft, and caused problems between her and her husband, considered a sin, and will she be held accountable for it?

If the situation is as you described—that the woman and her daughters transgress against you with insults and physical assault without justification, accuse you of practicing sorcery without proof, and cause discord between you and your husband—then all of this is aggression and injustice. Your boycotting them to avoid this harm is permissible. Al-Hafiz Abu Umar Ibn Abd al-Barr said: "They (scholars) have agreed that it is permissible to boycott for more than three days for anyone whose interaction brings a نقص (deficiency) to the interlocutor in his religion, or causes harm to him in his self or his worldly affairs." We advise you to pray and trust in the reward Allah has prepared for the patient, and that your right will not be lost as long as you are oppressed.