Stillpraying,
I think it's hard for many people to acknowledge the bad qualities in others as also belonging to themselves. I have a friend that is absolutely going through a MLC herself right now. Last summer she got a "tummy tuck" and a hair piece. In the fall she left her H of 20+ years. She met a guy on a dating site and hopped in his truck on the first day they met, had relations with him within 1-3 days and moved in with him on the 3rd day. She's also changing jobs now. I should add that she went through her "religious spell" during the winter. I guess she gets a check mark for all 4 pillars:)
Many times throughout this she has pointed out the horrible things that my runaway has done but then qualifies everything that she has done as different and that her marriage was bad for the past 5, 10...oh wait...now it's been bad since before it even happened.
I suspect that your sister is probably along those same lines. She probably doesn't talk about it at all either out of shame. IF they never did any counseling or anything afterwards the problem will probably be repeating itself.
J.