What exactly are you saying when you expose to his parents? I get the impression they have the usual ostrich syndrome (bury their heads in the sand...).
I would meet with them AND his siblings w/o your h there... have a family meeting and sit down to discuss what to do about the marriage.
I know they don't want to get "in the middle", but what they mean is they dont want to "feel uncomfortable"
They aren't getting in the middle, you don't want them to play referee and take sides.. you want them to side with marriage over divorce...
That means they stand up and say BOTH OF YOU.. GO to THERAPY, stop CHEATING and stop LYING and MAKE IT WORK
That's how parents should be handling things, not hiding to spare THEMSELVES some discomfort
