KSacres--I used to think that way, too........until I had kids. Tell me this, if something does go wrong in your relationship, who protects the kids? Marriages should be high priority and it's really great you put your marriage as top priority. I try to, too. But, kids are very needy little buggers and sometimes they simply have to be put first. You can love your spouse like crazy, and then develop a crazy strong different kind of love for your child. And, this SD was once a little child, probably only about 10 when dad remarried.
Regarding the OP, the whole tone drives me nutty. Slapping her upside the head? Symbolic cuss words to describe her? Yes, children should be respectful. So should adults. Yes, the husband could step in and try for some great communication. So could the step-mother. Why set yourself up for evil stepmother versus evil stepdaughter? You are BOTH fighting over this man. Why? He is your husband. He is her father. Where's the conflict? Can he not be a loving husband and father at the same time?
I think some really mature conversational skills would be very helpful here. I also agree that simply refusing to fight might be helpful. And, as far as I know, a person still has the right to invite whomever they wish to their wedding. I guess your husband could refuse to go since his wife is not invited. Or, maybe you could acknowledge that your being there might be a very painful thing on a really important day. Since your relationships sound very adversarial, I guess I can understand that.
Good luck!