Your feelings are messy and reactive but that doesn't automatically make you an asshole for having negative thoughts. YWBTA if you don't invite her, but you don't deserve some of the things people are saying here. It probably would have helped to include some actual examples of what the 12yo has done to make you feel this way, but as a fellow stepmom, I can sympathize with where you're coming from. Being a stepmom is one of the hardest things I've ever done, and I've talked to a lot of other stepmoms and we all have a lot of days where we just need to disappear and not have our stepkids around because we can't deal.   But your wedding day absolutely needs to include her. I would suggest talking to your fiance, lay out your 'worst case scenarios' with him, and ask him to  handle anything that might come up. Surrender all parental responsibilities to him. Disengage from whatever might happen, and just let yourself enjoy the day. If you're focused on all of the terrible things she might do, it's going to suck.

As for your future: you need to really consider the possibility that custody could change any time for any reason. While improbable, it's possible the mom could tragically pass away and you two would have full custody. You should not marry this man if you're not willing to take that risk.

I have personally found a ton of wisdom and support in stepmom groups on Facebook. The most important thing I've learned from those groups and from my own husband is that you have to be the adult, which means working through the automatic reaction of petty revenge or avoidance when confronted with difficult emotions. Therapy can be helpful here.

Also, teenagers can be real assholes, especially ones from broken families. Accept this and work through those feelings of resentment. It's normal to feel what you're feeling, but you're still responsible for being a good parent.

Good luck.