Is a brother entitled to object to his father’s supplication for his brother who is on the verge of death, and what is the father’s share of complaining about him to God?

The question is unclear. If the father prayed Dhuhr on behalf of his son, the prayer is invalid, as vicarious performance is not permissible in prayer. However, if he prayed for himself and then supplicated for his son, there is no harm in that. As for your disapproval of his engaging in forbidden matters, there is no harm in that, provided you maintain respectful communication.