My mother in law bought a home in 2011. In late 2012 she started begging us to buy her home from her by just paying off the house, she wanted nothing just to rent from us until she wanted to move. We finally agreed and did her wishes in 2013. All transactions were complete through a sales contract, deeding it over to us, and us paying off her loan. Now her sister has found out and is trying to make us sell it back or merely quickclaim it to her. We are paying back our own loan. Can they make us sell?

No, the sister cannot force you to sell the property to her or anyone else. The only way you would ever be forced to sell is if they can prove that you defrauded your mother-in-law. Based on what you've said here there was no fraud involved.

Whose name is on the deed and mortgage? The mortgage was assumed by you through the mortgage company? How is the sister involved in ownership of the house?

Ok, so entered into a contract with your M-I-L to buy the house. Was it for fair market value? The contract specified that you would pay off the mortgage, right? Did you pay anything over to your M-I-L?

Once the contract was signed, your M-I-L deeded the property to you and you paid the mortgage with a mortgage of your own? Or was this all done at a closing?

As Lisa said the only way they could force you to return the property is if fraud was involved. And if you didn't pay market value, that could help. But if the M-I-L is of sound mind it will be very hard to prove.