ANSWER:
He’s sinful for not gifting equally among all his children while he’s still alive. However, this doesn’t give you the right to be disrespectful with him at all. He’s sinful for his mistreatment among his children and would be answerable before Allah and you would commit a major sin if you cut ties or be disrespectful with him. Do you want to sell your father for such issues? This is too cheap for a muslim to deal with his parents in such a way!