What is the ruling on a sister boycotting her brother and his wife who are harming her, given that she can no longer tolerate their actions?

Your patience with the harm inflicted by your brother and his wife is better and earns a greater reward from Allah than estrangement and abandonment. Indeed, the one who maintains ties of kinship initiates the connection, even if his kinship ties are severed. However, if you are harmed by associating with them, there is no sin in abandoning them with a beautiful abandonment that does not incur sin upon you. This beautiful abandonment means avoiding association without harshness or harm, for beautiful abandonment is better than harmful association.