This is why if you expose, you should expose to everyone SIMULTANEOUSLY. Otherwise they try to spin it before you tell the next person.
Your SIL is going to support her sister -- "blood is thicker than water" is so true. Just tell them "I decided I was no longer willing to lie to cover up (wife's) affair. If you all don't want to deal with the truth, that's up to you, but I'm tired of lying an enabling her infidelity. I love her, and want very badly to work on our marriage, but this is just blatant disrespect, and I have to protect myself."
Or some such.
She WOULDN'T have been honest, with any of them. ALL CHEATERS LIE.
