Awkward situation: Want to invite mum to wedding, but she'll bring horrid step-dad...?

Don't lose sight of what is really important here. You and your fiancee want to get married and live happily ever after. Starting your marriage off with a family feud is a really rotten way to start. You should probably tell your mother that your stepfather is not welcome at your wedding. Your mother will probably object but tell her that the only other option is that you are getting married in either a civil ceremony with no one present. You are going to be dealing with a mad mom for some time to come no matter what you do so you are going to have to make it plain to your mother that the two of you don't have any good choices, only less bad ones.