Cynthia and Ann
There is a rivalry between them since young, that's what I feel but why? Cynthia probably feels jealous that Ann has a "stable" life like a nice house and a husband and that the future is paved for her which is why I think she slept with Ann's husband. Also Cynthia is a sexually active person which would help in that cause.
But I don't get how Ann easily forgives her, is their disgust with John a common thing that they finally share (besides plants) that they have finally felt a bond?