When is your Disney wedding?
How old will the baby be?
I did not have any babies at my non-Disney reception because none of our guests had babies at that point.
On the other hand, my husband and I were in our friend's wedding when our oldest DD was just 4 months old. They told us we could bring her to the wedding if we wanted since there would be other babies there. I was breastfeeding so it certainly would have been convenient to take her. In all honesty, at that age it would have been very easy to take her since she pretty much slept when she was amongst a crowd of people. We instead opted to take her to the rehearsal & dinner the night before and let my mother take her for the day of the wedding and have a nice night out. She adamately refused to take a bottle until she was starving, so I of course was worried about that all day!
At my sister's wedding there were three babies present, a 3 month old, an 8 month old, and my "baby" 2 year old. The babies were fine, a bottle (or modest breastfeeding) or a pacifier kept them contented through the ceremony when they were awake and they could sleep through anything. The only one who had to leave during the ceremony was my 2 year old. Fortunately, we had planned for this and my MIL attended the wedding fully prepared to take care of my daughter.
While the decision is ultimately yours, maybe you could run it by your sister and get her thoughts on it. Personally, I wouldn't be comfortable leaving my baby with a non- family member babysitter.
but I definitely can't see myself telling her "no baby" either. Hopefully her husband's mother could watch the baby while she's at the wedding. There is plenty of time to decide this but it's been heavy on my mind. 
