I rent a house, my sister in law's sister needed a place to live, we talked through email and she agreed to pay rent. She moved in August to our home, she has not paid a penny. I gave her a letter stating that if she did not pay by December 15 she would have to move. She has not paid and is not attempting to move her things out. As I stated I am a tenant and I did this without the owners knowledge. The letter I gave her gave her 30 days notice. Can I change the locks on her on December 15?

No, you will have to file for official eviction in housing court.

They are doing a sub-rent. And you are their landlord at this point.

Never ever change the locks without a court order. If she has never paid any rent then you can immediately file a lawsuit for eviction for non-payment.