Help! Family Party and Can't Invite Cousin!

Hi Moms,
Well, there's one in every family, and I certainly have a nutcase in mine! I have a cousin (she's 25 years old and we are 2nd cousins). I hate to say this, but she's always been a bad kid. She's stolen, she's hurt people that she's worked with, she's a HUGE liar..... And as she's gotten older, she's not any better, just sneakier. About 6 years ago, she stole a beautiful watch from my mother's house at a family party. It was very sad because it was an anniversary gift from my dad to my mom. And we are POSITIVE that she took it (my mother saw her sneaking around her bedroom at the party and afterwards, we found out the watch was gone). There have been MANY other problems with her and other things that we know have been stolen by her. But my mother never said anything because she and this girl's mom are 1st cousins and very close. And my mom knows that if she told the mother that all this happened, she would get angry at us and never speak to us again. (part of the reason that her daughter is such a bad kid is that her mother gives excuses upon excuses and keeps a blind eye to all the behavior problems). Anyway, we were just putting up with it all these years, but this past week I had a large family party at my house. This cousin of mine was the last one to leave and we (me, my hubby, my parents and brother) all noticed that she kept hanging around the kitchen. All of the sudden, she quickly bolted out the door barely saying good bye to us. We thought this was weird. Well, sure enough, my hubby and I were cleaning up and right before I went to bed, I smelled gas in the kitchen. Low & behold, one of my burners was turned ALL the way to the right. There was no flame, which means she had to blow it out. Plus, if someone else had accidentally turned it on, there would have been a flame and it wouldn't be turned as low as it could go. And it smelled all the way upstairs in our bedrooms. I am POSITIVE this psycho turned the gas on. Sooooo here's my problem. Since the issues with her have now become life-threatening (what if I would have lit a candle???) I REFUSE to have this girl in my house with my family ever again. But I have birthday parties for my children every year (and there are tot locks on my stove and my kids have never play the stove much less touch it - in case you think it might have been one of them). Since this part of my family is 1st and 2nd cousins, should I just tell the mother of my cousin what happened and live with the fact that we will probably never speak to them again? What do I do? I can't ignore this behavior. We did that with the stealing, but now it's much more serious. Is there a correct way of going about this? Please please help! Thank you Ladies!