That's really sad. But why are you trying to get him to admit to another affair? He's already been with three other women that you are aware of, one of them being your own sister. It baffles me why you haven't left years and years ago. You don't need him to admit to this affair, or any of the others - like you said he hasn't been interested in you for eight years, so why not leave on that basis - and find someone who isn't such an embarrassment.