Sil being a nightmare already

Probably best to get your fiance to deal with her as much as possible, so that you don't get branded as 'the horrible woman my brother married' for arguing with her!!!

I would suggest discussing and agreeing boundaries with your fiance and then just trotting out the same standard comments every time SIL gives you grief. Don't get into debate or argument - it's your wedding, not hers, so you don't have to justify any of your decisions. "Sorry you are disappointed at our decision to exclude children, but we are limited on numbers because of our venue." "Sorry you are disappointed by our decision to do x, but our plans are finalised and it's too late to change them." If she realises that she's only ever going to get the same stock answers from both of you, then hopefully she will give up.

As for the child's dress - you can't actually prevent her turning up in whatever her mother sees fit to dress her in. I would have your fiance point out that this dress would not be regarded as appropriate at a UK wedding, but not make a big deal of it. If SIL is genuinely open to doing the right thing, being told it's not appropriate will be enough to change her mind. If she's determined on having her daughter steal the show, the more you try to stop the kid turning up dressed as a mini bride, the more the mother will be set on that!

Ugh I feel for you. I've been bombarded with my family wanting to invite people that I don't even know. Literally had the biggest blowout imaginable because other family stuff for brought up. Lots of crying and avoiding talking about the issues! My finance came home and found me lying on the sofa, crying in the dark! Weddings are so unnecessarily stressful! People just forget its not about them. Its about you and your fiance! And that's it. Other people just need to smile and wave

"Other people just need to smile and wave"

I feel this needs to be printed onto a t-shirt for every wedding-planning bride to wear constantly!