My step kids bonding with their brother

Tete
My son is 3 weeks old and I am very uncomfortable with letting his brother and sister's hold or look after him when I am not around. My husband is though. He'll let the oldest who is 15 take him in her room with her and I can't stand it. He says I'm too paranoid and that I need to relax. We've been arguing about this for a while now because I keep telling him right now is not the time he's a newborn baby. They'll have plenty of time to do that later. I feel like right now is the time for him to be bonding with his parents period!! Besides that I don't trust them enough!! Am I over reacting??