WIBTA if I told my sister she can’t bring her two kids to my wedding because the venue has no space for them

I’m planning my wedding for next summer and the venue is a small barn that only holds eighty people. When I gave my sister the guest list details she immediately said her kids, ages four and seven, would be there too. I reminded her the invite was addressed only to her and her husband, and that we’re keeping the guest count tight because the space gets crowded fast. She told me it’s not fair to exclude family and that she’ll have no one to watch the kids since her regular sitter is also invited. I offered to help find a sitter near the venue but she said that’s not the point and that I’m prioritizing strangers over her children. I’m leaning toward holding the line to keep the wedding comfortable and within capacity, but I’m worried this will cause a huge blowup right before the big day. I get that she wants the kids included and that it feels personal, but adding two more seats means cutting two of our friends or moving the whole thing to a bigger place we can’t afford. Would that make me the asshole?