Selling car 2 family member

What is the name of your state (only U.S. law)? PENNSYLVANIA

**I AM PRETTY SURE THIS IS THE WRONG BUT COULDN'T FIND THE RIGHT ONE *** SORRY


I recently sold my car 2 my sister. Yes I realize now it was a bad idea. SHe was in a bind and needed a car & i was selling mine. She was unable to get a loan so I left the loan in my name. This was on October 1st 2008. She now has come to me and stated that she cannot afford the payments & wants to give the car back. I know she has the money we had went over a budget.
I printed out a buyer & seller agreement online & added a few minor details that were specific to our situation. This was signed & taken to a notary.
If my sister chooses not to pay will this hold up in court or am I fighting a battle I may not win.

the document says that she will agree to pay the payments until the end of the loan & the car was sold as is. She is also on my insurance since that made her rate cheaper.