Oh, and, back onto Cassie; so, I was a little curious and did a little 'research' (emphasis on little: I very carefully asked an AI... lol). If anyone else is curious too, it seems like incestuous attraction boils down to a few potential triggers, assuming the AI was on the right track:
1) Isolation - Emotional & social isolation; so the MC would become the main source of unmet needs. (Cassie sounds like the opposite here)
2) During development: role confusion, unmet needs, or inconsistent boundaries - So, if both Cassie and Guy had unmet needs from their parents, Guy would've had to become a surrogate parent to Cassie, and possibly other things as well, like a friend. This can potentially cause 'role confusion' for both Guy and Cassie, which could lead to blurred boundaries. I think here it would just be role confusion between parental figure and sibling, though, not romantic partner.
Also possible that the father was sort of involved in the family, maybe especially when Guy left. If that's the case, the father could've created role confusion for Cassie as well, and that might bleed into how she perceives men in her family, like Guy? I don't know, I'm no expert, so I'm not gonna pretend like I fully understand how it's supposed to work, because I definitely don't.
3) Personality & attachment style - Borderline and narcissism are associated with poor boundary control and dependency, and insecure attachment style may predispose someone to unusual relationship dynamics. So, maybe these are factors for Cassie?
4) Genetic Sexual Attraction - If Cassie didn't grow up with Guy for long enough, it's possible she never developed the 'westermarck effect' (biological/psychological repulsion) towards Guy. So, maybe she associates him with a caretaker that met her emotional needs, yet not as sexually repulsive because he wasn't around long enough?
5) Disregulation of sexual identity/self-schema & history of sexual abuse - So, if Cassie was sexually abused when she was younger, especially by a family member, then her boundaries and sexual identity could be warped.
That's all I got. Not much, and I feel pretty weird focusing this much on the theory of incestuous attraction.

But, it was morbidly interesting!