Short sale that is tenant occupied by owners sister

Hello,

I'm about to sign a purchase contract for a home that is a lender approved short sale. The property is currently tenant occupied by the owners sister who prefers to stay put. This sounds like a great investment opportunity for me since I was going to rent it anyways. My concern is she may be living rent free or very cheap. Is there any steps I should take to protect myself from getting stuck or make it contingent upon her eviction?