UPDATE: A lot of you absolutely saw this coming but after we had a deeper discussion about this it turns out he's a fence sitter - and has been since we started dating 5 years ago! Said he's leaning more towards kids since his sister has had her 3. He figured I'd not have given him a chance in the beginning if he told me since I was very up front about being childfree so he lied and believed I would change my mind eventually. He even went as far as to say 'WHEN' I change my mind rather than IF I do. Needless to say we will be separating and he will be moving back in with his mother. This has been such a punch to the gut but thank you everyone for taking the time to comment and give your thoughts and advice.
My partner's sister has 3 kids under 5, they'll be moving to our area soon. We are both childfree but I've always had stronger feelings about having less voluntary interaction with kids between the two of us. I've always wanted our home to be a childless space which has never been an issue as his friends with kids and sister have always lived quite far away. He feels that he wants to be able to host his sister and their kids and help with baby sitting here and there and thinks I'm being unreasonable. That his nieces and nephews aren't some random kids and I should feel differently about them and be okay with having them in the home and 'bending my rules'.
Has anyone else had experience with this? How did you get through this, or was it a deal breaker? I suggested anytime he wants to invite them for dinner he could book somewhere outside of the house for that, or babysit at their house but he's not keen and doesn't want to have to tell his sister that the kids aren't welcome in our home.