How to tell a family member not to come to an event?

She's about to be my sister in law. She has caused nothing but cause drama with most members of my family. I personally have no problems with her as I've only known her a short while & we are on friendly terms. My mom is having her annual party for the 4th. She assumes since she's about to be my sister in law she's automatically invited. Well whatever right. So no my mom finds out & b/c of the problems she's caused with other close family members she doesn't want her there & wants me to tell her. I said no & told her to do it. She's now laid a huge guilt trip on me about helping her out in a sticky situation & thinks since I'm her friend that it would be better coming from me. My mom's insistent that she not come (along with several other family members) but I don't want to do the dirty work or cause any further drama. What should I do?