Then your sister is a thief. She entered into a contract with the lender and is obligated to abide by the terms of the mortgage.
She’s just playing their game.
I seriously doubt the contract gave her the option. The “option” to give the property back is called being in default. The property was merely collateral. It is legal in the sense that the bank can take the property back to cover some of its losses. It is not legal to the original terms of the mortgage except as a safety net for the bank in the case of deadbeat customers who default on the agreement. Sounds like your sister is even taking advantage of that and living in a house she is not paying for. Shame shame shame!!!!
The “or give it back” condition applies to leases, not mortgage purchases. Was it a lease purchase? If it was, even then the sister should be paying a payment while she lives there.
Legal is not a relative term meaning “whatever you can get away with.”