Babysitter at a wedding?!

It's only bratty if you said it bratty-ly lol. There is nothing wrong with a kid free wedding, and they absolutely have enough time to figure it out. I think having a baby sitter at the venue is equally annoying/problematic as just having the kids at the wedding. The kids would still likely sit with their parents during dinner, and not all of them would be very good at staying in the designated room the whole time (unless they are all older children maybe). You have the type of wedding you want!

If you want a kids-free wedding, you need to stand your ground on this.

But heads up: breastfeeding babies (so probably her SIL's baby) are usually exceptions to the no-kids rule, since it's much more difficult for them to be away from mom for very long. Babies (obviously) won't run around and don't require an extra seat, so I'd say that should be an exception. Other than that, continue to hold your boundaries and stand firm on the no-kids thing.

I would just stick to your guns. No kids. Even if you hired a babysitter, you couldn't force parents to use the sitter.

Not bratty at all. It's not your job to hire a babysitter when they have 8 months in advance to find one! If the size of the venue wasn't the only reason that you decided not to invite kids to the wedding, then it shouldn't be a problem to keep them off the guest list now.

I hired a high school girl to watch a few kids (not babies) at my first wedding. Only my family from out of town used them but it was good as that way the parents are also able to enjoy the wedding. Others hired their own and left early.

Nope. You are not being bratty. It is your wedding and you decide who is and is not invited. If the parents of the baby want to go, they need to figure that out (and as you put it, they have 8 months to figure it out). Take it from a parent, those are events that we look forward to because then we don't have to worry about our kids and have fun, adult conversations (and drinks).

My husband and I got married in October, and we had my stepson (ring bearer) and nieces and nephews (because we wanted them), but no friend's kids or extended family. We did all of our inviting in groups, so after first cousins we ended the familial invites. This caused a major rift between my mom and I (who agreed) and her sister (My aunt) and my cousin. My cousin lived about 6 hours away, all of her invitations clearly stated it was for her and her husband, not the family, and yet she would talk about how she got a hotel room for the whole family and what she was doing with the kids. I had to let her know (awkwardly) that her kids were not invited. She immediately responded saying she can't make it, my aunt no longer talks to my mom and it's a whole thing. I still stand by my decision; if I invited her kids, I would've had to invite about 30 other people that i simply couldn't afford.

Your wedding is not anyone's family reunion, nor is it their babysitter. People should do their own bidding on that end.