Brother's girl, I can't stand her

I'm having a problem with my family right now and have no idea what to do, or where to go. My 17 year old brother's 19 year old girlfriend is living at my parent's house because her "parent's are evil and kicked her out, and her grandparent's just won't allow her to live at their house" so she has no where else to go. Now, she's been angling to live at my parent's house since they began dating about 9 months ago, but my parent's firmly said NO to her. But now, she's managed to worm her way in, and I doubt she'll ever leave. I'm 23 and married, which makes me think that I must be crazy for being so upset about this, but she's in my old room which I shared with my sister. She has been cleaning because she is supposedly sick from dust under the bed, but it is making me so upset that she keeps touching my things and being in my old room. She's living there with no rent to pay because "she needs to save so she can move out." That is such a load of crap! My husband and I both can see that she's using my family because she has an unhappy home situation and wants to live there. My parents have always taken care of my brother's loser friends in the same way that some people take care of stray dogs. I probably sound like a selfish jerk, and if I do just don't comment...but I swear this is a bad situation and I just don't know what to do. If anyone has any ideas of how to cope, please let me know. I'm losing sleep over this...Thanks.