It wasn't so much "cheating" as it was a one-time backslide. She, like D, had an affair with her Dad when she was still a teenager, but then she broke it off and started seeing F. Her Dad then came to visit her at college a year after she and F started dating and she succumbed to him that one time but it didn't happen again after that. She knew it was wrong and regretted it ever since until it became too much for her to keep lying to F so she did a rash thing and ended their relationship.
It doesn't necessarily make it any better, but I actually gained a lot of sympathy for her, not just because I want to see them have a family threesome, but she does seem genuinely remourseful for her actions and I believe both D and F should give her a chance to redeem herself.