Is a father entitled to stipulate to his son that he speak with his abusive sister without reprimand or any indication of how to deal with her in the future? Should the interest of one child be sacrificed for the interest of another? Is a son's relationship with his father tied to the children's relationships with each other?

The rights of parents are great and are not waived by their wrongdoing. Obedience to them is obligatory as long as they do not command a sin. Even if their command is difficult for you, as long as it does not harm you, you must obey. Therefore, you must obey your father in calling your sister and refraining from blaming her, because she is a kin whom you are obligated to maintain ties with, even if she has wronged you. Strive against your own desires and intend by your action the pleasure of Allah.