It depends on if the Church considers him free to marry. If he was properly laicized and dispensed from his vow of celibacy, then he presumably was free to marry his first wife and may remarry now that she has predeceased him. On the other hand, you have to weigh in the scandal he has caused by entering into a relationship with a woman under his direct spiritual care and authority, by leaving the Catholic Church, and by becoming a Protestant minister. Given his history, it is not unjust to wonder if his current fiancee is also someone who was dependent upon his spiritual authority.
If attending this wedding would appear to give approval to these kind of acts, then I cannot recommend attending this wedding even if he is otherwise free to marry. His family’s happiness over your choice should not be a factor in making that decision, but you should try to reach some kind of agreement with your wife over the issue so that the two of you can present a united front to her family.
