Is it permissible to continue shunning one's brother after the father's death, because the brother returned to his unfaithful wife, knowing that the husband prefers this shunning?

If the wife insists on forbidden acts and your brother approves of her, then it is permissible to boycott them. You should inform him that the boycott is due to this wrongdoing. If your connection with him is more beneficial, hoping for his rectification, then maintaining the connection is preferable. If your husband orders you to keep a distance from them without cutting ties, then obedience to him is obligatory, because kinship ties are achieved by the least action that removes estrangement. Continue to advise your brother and his wife, while observing their concealment.