Hi. If the agreement for the flat is registered in your name, then there is no way that your in laws can sell the flat to a third party. They do not have a registered document in their name and thus do not have a title to the flat for them to be able to sell the same. Yes there is one possibility of your in laws trying to illegally deposit your title deeds as a collateral security for any loan. But that is also not possible without your signature. Since they have snatched the documents from you illegally, you must immediately lodge a criminal complaint against them with the police.