Oh, and them mentioning or talking about you in the emails? Deplorable, perhaps. Definitely tasteless, unkind, thoughtless, cruel....
But it's not the fault of BDSM that they did that, it's the fault of your wife and her bf, and their shitty judgement. Don't blame BDSM. Blame your wife, and blame her bf.
I agree with whoever said this ISN'T polyamory, this is DYSFUNCTION. Stop fixating on what they did in bed. Focus on how they've wronged you, and the damage done to your wife's relationship with her daughter. There's been plenty enough of it.