1. You can file a suit for injunction against your in-laws to seek court's directions to them to return the documents of property of your wife.
2. Your in-laws have no share in the property gifted to your son by your late wife. Mere possession of title deed and other documents of the property does not authorize the possessor to sell the property. The gift deed cannot be cancelled after it is executed except if it is conditional. To obviate the possibility of illegal sale of property by your in-laws you may file a suit for injunction against them.