BN, I'm sorry this happened to you; I understand how painful that can be.
One of my sisters seemed to take an attitude w/me, partly how could I NOT KNOW when/if he was drinking for all those years? and partly is it really that bad? She is the sister I am closest to by far, so this hurt. Two things happened to help here:
1. RAH talked to her (his own idea, I didn't know until afterwards) and said he had not been sober at a family function in YEARS, and was SHE able to tell? And of course she said no.
2. I had this realization: She herself has a VERY dysfunctional marriage, and I am sure she has not loved her hub in many years, yet won't leave and won't talk about it at all. Since she chooses to live like this, maybe she is threatened by the idea of my changing my life/marriage?
Also, a number of years ago I "came out" about childhood sexual abuse by my stepfather. My mother refused to believe me, as did his children. Eventually I was able to understand that here, too, they were protecting THEIR reality and it really wasn't about me.
Don't know if any of this provides any useful insights for you, BN, but maybe if you consider the sources, you'll see reasons. NOT that that makes it OK or less painful, but so that once again, you can see that the behavior is all about THEM and not really so much about YOU.
MTA: Just saw dandylion's post, and yes, you have helped to portray him as the charmer he wanted to be seen as, and perhaps all you have to do is stop covering up for him. The truth will come out eventually, right? Let him sink or swim on his own.