Overcoming Infidleity explains the importance of exposing pretty clearly...
You need to stress the importance of the THIRD PARTY bullying your marriage towards divorce.. You can't point at her daughter as the bad guy or she won't cooperate with you.
Offside note here. Do you see a genetic pattern here? Not only is her DAUGHTER hiding info from you, now her MOTHER wants YOU to hide info from your WIFE.
I dunno... I would tell her mother you aren't comfortable lying to your wife and won't keep a secret. I would warn the mother but I would refuse to lie anymore.
All of this lying crap doens't help man... the truth really does set you free.
Its the OM that's got to go man... once he's gone your wife will be a LOT more cooperative to exploring things with you again... Until that creep lays off you ai'nt got a chance.