What should I do regarding my sister's husband, who expelled her and mistreated my father and my family, and despite my desire for forgiveness, I see them escalating their mistakes and becoming more arrogant?

Forgiveness and overlooking are characteristics of the people of goodness and righteousness. Allah Almighty has commanded them and assigned great rewards for them, so the one who forgives will find his reward with Allah. The Prophet, peace and blessings be upon him, used to forgive and overlook, and not repay evil with evil, and forgiveness increases a servant in honor. Also, forgiving the one who wrongs leads to his reform and transforms his enmity into friendship, as in the Almighty's saying: "Repel [evil] with that which is better; then abruptly your enemy will be as if he were a devoted friend." And it is necessary to remind family members of the importance of forgiveness and overlooking, as in the Almighty's saying: "And let them pardon and overlook. Would you not like that Allah should forgive you?" If self-reconciliation becomes difficult, one can seek the help of some people of goodness and knowledge to mediate. It is forbidden for Muslims to shun and dispute with each other for more than three nights, and the better of them is the one who initiates the greeting.