My parents have a complicated relationship. They've been together since before I was born but were never married. My mom was miserable and started dating this friend of hers he's really cool and I have a feeling they'll eventually get married. But my dad still doesn't know and is living with mother because our house got destroyed. He doesn't want us to live with another man because he thinks he'll replace him or some stupid ish like that. I've grown to really like this man although I haven't known him long and he makes my mom happy. But I feel like I've betrayed my father.
First, he doesn't own you or your siblings. Second, if he failed at being a partner to your mother, then he can't have a say in whether she marries a man she loves. If you like your mother's partner, then you like him. You can still love your father and have a step-father, too. If he's too jealous, then he needs to be put in his place.