Your wife has been complaining to your sister that something is wrong with your cock, you can hardly ever get hard and she isn't pregnant yet, maybe she should take you to the doctor. Your sister who is very possessive of you, will use that chance to steal you from your wife! You never had any problems when you were fucking her, the real problem is that you should never have gotten married to your boring wife. But just to make sure, she wants to run a series of tests in your cock! If you fuck her and cum inside her, then it means you are fine and you should move back in! If she gets pregnant even better!