What is the optimal course of action with a husband who avoids defending his wife in front of his family members and prevents her from socializing with them, especially after repeated negative incidents with his sister?

Since the husband has excused his wife from visiting his family to avoid problems, there is no issue. However, it is advisable to strive for reconciliation and to overlook their mistakes. Pardoning those who do wrong and overlooking the errors of others is a noble quality beloved by Allah, for which He grants reward and increases the honor of its possessor, as the Messenger of Allah, peace and blessings be upon him, said: "And Allah does not increase a servant in anything due to forgiveness except in honor." Responding to evil with good and using kind words brings affection and protects against the evil of Satan, as Allah Almighty said: "And not equal are the good deed and the bad. Repel [evil] with that which is better, and thereupon the one whom between you and him is enmity will become as though he was a devoted friend." This breaks enmity and accustoms hearts to harmony.