I think I'd probably avoid the blackmail and simply be very straightforward and direct. "You know, you set all these rules and ask me to follow them, but you obviously aren't governed by any true sense of right or wrong yourself." (Introduce Exhibit A, photographic evidence, here.) "I don't see any reason why I should have to follow the rules of someone who behaves like this, and you can expect that I'll make Dad aware of this situation shortly."