NTA = Not the Asshole.
Now for something a little “lighter”.
I don’t have a reddit account, but on reddit a lot of these kind of topics come up a lot and the “audience” let you know if they think you’re the “a-hole” or not. Why have I chosen this story? Although it’s not specifically about dating, it is about a situation between two exes.

Personally, I don’t think sister/ex-wife is the a-hole. But I have been in a similar situation, I have been a last minute wedding invite, because the original person never RSVPED. My ex was there with his new girlfriend. I went because we did have a lot of mutual friends in common, and I ended up sitting with them, while my ex was punished. So it suited me fine, haha. In all honestly though, if that person had contacted them and he could have gone, I would have not gone.
There is no reason to not un-invited people who ARE a-holes. Especially to your family, and if you don’t even know the people that well.
What do you all think of back up invitees?
I personally have no problem with them, or being one. You should always be careful though about having them, I feel you should be honest with them. You should also consider does that back up have an ex attending, do they get along? Do they have a partner, do you invite them too? Oh, it is not an easy thing these wedding lists.