Car broken into repeatedly by family, what can I do?

My fiance and I rent a room in her mother's home. We have a car that is currently non-operational, though we are working to change that (just needs some minor things done).

The problem is twofold.

1. Though the car was given to my fiance as a gift from a family friend, it is registered in her mother's name until we transfer it to one of our own names.

2. Also living in the house are my fiance's siblings. Her two brothers are the biggest potheads I've ever known, and have decided they can smoke in our car when it's too cold to do it in the backyard. Despite my complaints, I have caught them and their friends in the act several times, and I am finding trash in there almost daily. It smells now, and they broke my driver's side lock to get in.

So my question is, can I do anything? Am I out of luck since the car isn't in my name yet? Would it be too late to do anything after it's transferred?

I'm at my wit's end here and am nearly ready to just put a bunch of tacks on the seats. =\

Your fiance's mother is the owner of the property, owner of the vehicle and parent of the potheads. If she is unwilling to do anything about it, there's nothing you can do in terms of controlling their use of the car. If smoking weed is illegal where you live, you might tip off the cop that patrols the area to check them out when he/she sees them in the car.