I've been here. But I was the pregnant ex who got left...
All I can read from this is how over the top you are being!
From someone who has been through this and only wants what is best for her children, let me tell you how hard that is, trying to arrange contact with their Father in itself, let alone having an interferring fiance chirping up in the background!
My interests lie solely with my kids and how he is with them...but I can hand on heart say, if I ever thought his gf was the one trying to call the shots where they were concerned, it would make my blood boil and probably cause me to be difficult on purpose! You are not helping yourself, at all...
As it is and how I see it, you're a spare part in this. All I care about is that when my kids are with him, she treats them like her own.
But to try demand his DD be at your wedding? It might be your wedding too but that's between him and her, on how they're going to deal with that one! Pick ups, drop off, attendance etc - all them. She is their child!
Always is and always will be until DSD is old enough to make her own choices so if you can't deal with that now...how are you going to cope with this until she is a teenager?!
As for your Sister, I don't think she was being unreasonable. You must've known the shit storm you were getting into when you got with him, so yes, she's probably a little fed up of your woe is me speech.
She might be your DSD because you've been in her life from birth but you sound as though you have zero respect for her actual Mother!
You say you feel sorry for DP but that was his call wasn't it? He chose to live his life with you instead and he hasn't been refused access, even if it's not overnight?
I choose to let my kids go overnight because I want them to have him in their life but it doesn't mean it makes it easy!! My heart breaks everytime and they all stay together at his gf house - takes a strong woman to f*cking let that happen, trust me but you won't understand this until your baby arrives and you see how fiercly protective you become.
You really need to accept you will never have the deciding vote over what happens with DSD and back off, otherwise it's a disaster waiting to happen...and apologise to your Sister for starting an arguement over something that you shouldn't be adding pressure to in the first place!