SensualVixen: Also, we do not know if your brother-in-law had moved out before he hooked up with "the" mistress. There are too any "ifs", "buts" and "butts" to decipher in this thread from what little we know and many assume.
We don't know anything about your sister for that fact. We don't know if your sister, as a weekly ritual, dressed in black leather dominex boots, bra, and panties and dragged her husband down to the basement and shoved his penis in a vise and cranked it to "high" every Thursday night while eating a bag of of Doritas and shoving a candlestick up his ass. Maybe he got tired of black and wanted red boots occasionally??? We just don't know.....
She knew and requested counseling, and then caught him with her. They were still living together.
Once she demanded he leave, his girlfriend asked her husband to leave also.