What's the best way to deal with shady people?

I have a cousin that lives with me that is REAL shady. My mom went on vacation, and this wannabe homewrecker put on short shorts as soon as my mom left, started following my dad around, and went into his room at night with the pretense of "looking for tylenol". I already confronted her, and I'm planning on having her kicked out of the house as soon as my mom comes. In the meantime, I'm making her life miserable. I never dealt with someone trying to get inbetween my parents' marriage before. Especially a 17 year old cousin that is very shady and is trying to do my mom dirty behind her back. She's not even our responsibility, we're just trying to help her out. But she just screwed herself over.