With the current information, YTA.

You haven't bothered to have an adult conversation with your future step-mother, much less your dad. If your immediate response to this is "oh well she doesn't deserve my time" or something along those lines, then you're the one being a problem. Stop being a child, and at least try to talk to your stepmother. If she's going to be childish and refuse, then this isn't on you.

Also talk to dad.

Some advice for outside the marriage issue: try and think of why step-mom doesn't like you. Does she really just hate you for existing? If she was such a shitty person, why would your dad propose to her and have 3 kids with her? Your dad has probably also noticed the back-handed comments from her, and he hasn't done anything about it? Why?

Further, it sounds to me like your third-party voice of reason (real mom) has the same approach as I do. This isn't the right hill to die on. If step-mom really is just a shitty person for no reason, you should still go to the wedding. This is coming from someone who has gone through a family crisis that was caused by someone's immaturity and refusal to just talk it out. You're 16, man. Your dad, mom, step-mom are going to be people that you see a lot for most of your life. Why damage relationships and hurt feelings over this?