I’m sure I’ll get downvoted to hell for this, but NTA. But, that being said, I think if you don’t want to live with the kids, you need to have the courage to not move in with him.
I’ve been in your shoes before. I love my ex-stepkids very much, but one of the conditions I had for moving in with (and moving out of state with) my ex was that his daughter whom he raised alone was going off to college and living in a dorm. That lasted all of about one semester before he decided “his daughter needed to be with him” and made her come home. I was livid, and my worst fears came true. Instead of being a partner with my ex, I had to be the cook, maid, and caretaker of him, her, and his son who he had every other weekend. (Oh! And then she got pregnant, so we had her AND the baby for three years.). One of my breaking points before I left him was the day that I flat out asked him, do you really expect me to clean up after your kids? (They were 23 and 17 at the time, plus the baby). And he answered yes. There is absolutely nothing wrong with not wanting to be a stepparent! You just need to decide if he’s worth taking on that kind of responsibility. I don’t think you’re an asshole at all for being hesitant about it.