Maybe I don't don't understand the full extent of your situation. I thought the issue was with the BIL, not the BIL and some other guy.
The initial divorce threat was when my W was thinking of running off with the pet shop owner. After finding out about this PA, my digging uncovered a very long lasting EA with my BIL. The pet shop guy broke off their plans to be together, bu apparently my W is still pursuing him over a year later.
M 17 years 3 kids EA start 2010 ILYBNILWY 1/2014 PA 1/2016 Bomb drop 2/2016 Renig on Bomb drop 4/2016 Living as roommates, EA continues
