How can one deal with the husband's family who have wronged the wife and caused her suffering, even though the husband asked for forgiveness and was forgiven, yet the harm from his family persists?

Beating the wife without a legitimate religious reason is not permissible, and it is a sign of ill-treatment. You have done well by forgiving your husband, as that is among the noblest deeds and greatest acts of worship. If your husband's family has slandered you, your father, and your sister with calumny, then they are committing a clear and manifest sin. You should forgive them and join the ranks of the pious who pardon people. The husband must protect his wife from the harm of his family and advise them gently, reminding them of Allah.