Woah. Cheating with your wife's brother's wife? Yikes. Not only can I not believe you did that, but I can't believe your brother-in-law would condone such a betrayal of his sister.
This whole situation has dysfunction written all over it in seventeen different languages.
I may be reacting very strongly. That's because I have no compassion for cheaters. The fact that this involves your wife's brother risks tearing apart their entire family. Super not cool.
Forget about a polyamorous relationship with your wife, at least for now. After what you've done, it will take a lot of work to have any relationship with her at all. Get down on your knees and plead forgiveness.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker