If you look at your situation objectively, your wife has done everything she can to destroy your marriage; she's having an affair with your BIL and she holds you in contempt. But, you still want her and you want your marriage to work. She has no motivation to do anything other than what she's currently doing, which involves eating a lot of cake.
The dynamics of your situation are the opposite of what should be happening. She should be begging you to come back because she's the one inflicting all of the damage. What can you do to change the dynamics? The DB response is that you need to work on yourself (become the man only a fool would leave) and stop serving cake to your wife.
I don't think those two things are mutually exclusive. I believe part of becoming a better man is providing clear communication about your expectations and boundaries while your wife is living in your house. Of course, that's not easy. It requires growing a spine and being tough.
