I exposed to my FIL and MIL early on, but it didn't change anything. The first time I told them (in January) they just said, "well this is just shocking!" And left it at that. Never talked to H like I thought they would. The next time (In April), MIL just went on and on about what a big jerk H is and he should have never done this to me. The the last time (In May) she said he was an SOB. But they never did talk to him about it. Still...I am glad that I exposed it to them because they would have to had learned sooner or later and H sure wasn't talking to them.
You do need to make sure that the information you have is accurate before you tell anyone.
Good luck!
Me-31 H-38 M: 5 yrs T: 7 yrs No kids Went to Prostitutes 10-1-06 Found out about OW 12-24-07 Bomb on EA/PA: 1-2-08 OW ended it with H "for good" 3-8-08 OW is back 4-19-08 H and OW tell me that they are in love 5-19-08 Filed for divorce 6-5-08 Divorced 7-2-08