Had a similar, but not as bad, experience w/my wife's neice from Texas a couple years back. After she took my wife's engagement ring to high school, lied about it, and damned near lost it one day (not to mention lying, stealing from my daughters, skipping school, etc.) we had the advantage of having the option of shipping her back to her worthless mother in Texas.

Once she's 18, you can have her arrested for trespassing, no? Or have her declared an emancipated minor and do it now.

Not pretending to have anything useful for you,
Mikem