What's worse?
Sleeping with your (frigid) wife's sister, or sleeping with your sister's husband.
Or, to change the sexes around, to sleep with your (impotent) husband's brother, as opposed to with your brother's wife.
I find it very odd that Ann easily forgives her sister, as if she'd only caught her scrumping a slice of pie from the fridge, whereas her husband John is depicted as a monster who finally gets his rightful comeuppance. This is especially strange since the movie tells us that Ann had always been frigid (unable to have an orgasm), so John at least had a motive to sleep around.
Cynthia's only motive seems to have been jealousy towards her own sister.
So in my book, Cynthia is the greater villain than John.