Kungfu I hope your wedding goes well. 
You've every right to want your day how you want it. No one is entitled to be at your big day unless you want them there.
If you've already decided to tell your sis she is welcome at the evening party with here family, I totally agree with that and that is what i would do in your shoes. Also, if you can I'd try and meet her for a cuppa, unless she lives miles away. I'd apologuise for overreacting and saying she could not come to the party unless she can to the service. I;d remind her what she said "... my sister agreed at the start she was fine with the plans her words " would be nice to spend an hour alone with the ones I grew up with..." And I'd tell her how much I wanted her to be there, both for the ceremony and the party but that whichever she came to, I would be delighted to see her.
I'd probably try not to get too aught up in comments on her partner, he sounds awful and now they have a baby together he may be in her life for a long time, although hopefully she will see sense and things will change. You and your mum and other siblings will be a lifeline for her so try and be loving and close and not allow this awful man to spoil either your special day or your special relationship with your sister.
Mayby the fact she wanted photos on her kids on the special day may be something that lures her back, but of course really it will be because she cares about you.
I agree with Across the pond pages back, position him in photos where you can photoshop him out and whatever else good advice she gave.
Good luck, enjoy your special day.