@LadyMrsLady: Ok, here goes:
I can understand her side of things – and no, I am not a mom. It’s an important event in your lives and the lives of your ILs and I think it’s only natural that she would want to take part.
I also understand you and your FI’s side as well. It’s an expensive event to hold and you should be able to hold the type of wedding that you want.
That said, I think there needs to be understanding on both sides. She needs to understand that this wasn’t a personal slight (and I’m assuming here that it wasn’t) and that you guys did what you could to accomodate their situation. You, on the other hand, need to be more understanding that things are not always as easy as they appear. As many PPs pointed out: not all babies will eat formula, many parents are unwilling to allow a stranger watch their child alone without approving of them ahead of time, and some kids simply can’t tolerate being watched by strangers.
So, what can you (personally) do moving forward?
1. I would start by not referring to your FBIL’s kids as brats – even on this forum. Depending on the amount of personal information someone posts on here, it could be relatively easy for someone to figure out who the poster is. Not only that, but it’s possible that even using the term a little online could cause you to slip up and refer to them that way in person. . . which definitely won’t do any good.
2. At some point, I would recommend one of you reach out to Future Sister-In-Law and apologize for any confusion or hurt feelings. I know that you don’t feel like you need to apologize and you have done what you feel you can to help solve the problem. . . but they are going to be your family soon. You are stuck with them for the rest of your lives. Apologizing shows that you both care about their family and that you want to have a relationship with them.
3. If you don’t want to apologize, then I would at least reach out at some point to discuss what happened or to try and resume a more normal familial relationship.
4. Enjoy your wedding.
And just out of curiousity, how did you feel about Future Sister-In-Law before you guys got engaged? I know you said you only met her once, but did you like her? Was she nice? Did she seem friendly? I ask because weddings heighten people’s emotions and make anyone, even the most reasonable person around, act unreasonably. That goes for guests and the couple getting married. . . and I’m curious whether the wedding planning is coloring the way you see her and the kids or the way she views you and Fiance.