No, once something is gifted, that gift may not be revoked, or compensation sought for it after the fact. If something was gifted to you, you don't have to pay for it unless you want to. The time for your sister to ask for money for the house was before she gave it to you. That's the law. Practically, bear in mind that if she is prepared to lie, your sister can file a lawsuit claiming there was an agreement to pay her for the house and you reneged. Without written documentation, she is not likely to win, but you can't rule out her winning, if she does this and is very persuasive and/or creates credible fake documentation. So while the law is on your side, bear in mind that you could potentially face a lawsuit that you'd have to take seriously and defend against.