Wuzlovesdubs, this story seems to repeat itself over and over, they must have a secret manual on how to end a marriage and still look like the innocent part.
Personally I think he may be having a very physical affair with this sister, I have seen elders doing that for years while giving very nice talks at the hall and fooling everyone.
You are dealing with an hypocrite here, act accordingly.
Enjoy the rest of your life, and give him absolutely no explanations. If he harass you in any way, call the police.
You will be much better off without him. All the best.