OP - You are in the very early stages of pregnancy, step back from the situation with the ex as much as you can as the stress will do neither you or the baby any good.
The wedding - Having DSD overnight at the wedding poss isn't a good idea, you will both spend most of the day watching an over tired, over stimulated toddler who will be surrounded by lots of people she doesn't know. I think the ex is probably doing you a favour wanting her dropped home. Let her be a flowergirl, and in all the photos, and for the wedding breakfast, then arrange for someone to nip her home. You can then enjoy adult time as a wedding, shes been included - everyone is happy. Is MIL really not going to drink so she can look after DSD? Pick your battles.
Don't have the ex there if you don't want her there. I have a feeling she wont want to be there anyway and watching the father of your child marry someone she will see as the OW (whether shes right or not) will not be nice, and after a drink or two, be the makings of a kick off.
50/50 is sometimes seen as a status, almost a tick box to being a good dad. At 2 its little and often. It doesn't matter how often, as long as this little girl knows her dad is there for her, loves her, and on so and so day she will see him. Overnight atm isn't the be all and end all. Quality is more important than quantity. If your DP can show that even though he is married and has a new child nothing has changed she will no doubt start to change her mind re contact. Don't let things escalate to the point it ends up in court, that is an expense, heart wrenching mess no one wants, especially DSD. Take whatever she offers and try to show her support.
You are about to become a parent. Focus your time on your LO. Let DP step in and deal with his EX. The more you show involvement, and the whole 'we' thing will just wind her up. I'm not against you or saying theres right and wrong, but when your DSD is older you and DP need to be able to say to her truthfully
- We loved you and did our best for you
- We supported your Mum as much as we could
-We always wanted to see you as much as we could and never let you down
- You have a home with us too
- We have always been a family
Hours spent together doesn't equal a family, its love, consideration and effort you give each other that does that.
Arrange for step daughter to go home after the speeches and enjoy your wedding. Tell DSIS, shes had her opinion, you didn't find it helpful or correct, but shes entitled to it anyway. Lets agree to disagree, and that as your pregnant and about to get married, lets agree to not talk about it. And leave it there.
NB - If DSD mum gets the hump, she may well just be giving you lip service saying she can go, then on the day or the night before, she gets 'ill' or she decides she cant go. Be prepared for that to happen if you keep pushing the issues and rowing with her. Get DP to have a chat with her by himself, saying the most important thing is that DSD is there, he doesn't want her looking at wedding photos in the future where she is missing. Say someone will drop her back.
How is she getting ready in the morning btw? Assign someone from DP family (not yours wont help the current row) to pick her up, watch her on the day, drop her home.
Enjoy your day