I feel for you, i have heard a few scenarios similar to this. It is hard to know who to include in an important event that can sometimes be quite a while away. Things to change!
I think it is your day and instead of worrying yourself you just need to be honest with her. You can still tell her in a polite way! You've already said that after the wedding you won't see this person so if she does take it really badly then you'll just stop being in contact with her sooner rather than later.
But also, perhaps her not getting back to you or putting in an effort is because she also does not want to be apart of it but does not know how to tell you!
I think if you called her and just said that you feel like she's not giving you the effort that you hoped she would and that you don't think the two of you have a strong relationship as you first thought. Honesty is the best policy! And really you can't waste time trying to get her to cooperate as you have too many things to organise and need to get on with it.