I am purchasing a house that is currently occupied by the sister of the owner. The sister pays no rent and is obstructing all attempts to get her to move. Owner lives in another state. The house is in FL. Who's responsibility is it to use the legal system to remove this woman from the house so that I can take possession the day I have the closing? The local realtor has a POA to act for the owner. I need to be able to move in quickly after the signing. I know that the sister in residence has been told more than 30 days prior that the house was being sold. She obstructed the showing, the appraisal and the home inspection. Says she has no place to move to as if that is my problem. What do I do now?

Once you own the property, evict her. Until that time you have no legal standing.

Does the sale contract require that the tenant leave prior to closing?

Do not even think about closing on the purchase of the house until the current owner has evicted the sister. You are entitled to have the house delivered vacant. If you close while the sister is still there then it will be your responsibility to have her evicted and that could become a nightmare. Getting rid of the sister is the seller's problem, not yours. Don't give them your money until the place is empty.