Since multiple people have mentioned that one pseudo-jealous comment from Rita, it has stuck in my mind now as well.
This is my idea for a story that pulls at that thread. Whenever Melissa masturbates, her thoughts keep coming back to that day, watching the two younger girls in the throes of passion with the two men and experiencing the "little death" with the big one. Soon it becomes a nagging obsession at all times, and she ends up seducing her father into a night of forbidden passion. Somehow, Rita finds out, doesn't think she can reconcile living with the truth, and brings up her ultimatum. Feeling some remorse for her shameful lust & a sense of familial duty, Melissa agrees to let herself be butchered (perhaps along with further wanton indulgence with one or both men, maybe one/both of the wives gets pulled in?). This way the semi-consensual/reluctant angle gets some exploration and helps give a unique flavor (both in tone/themes, and the meat).
No matter whether you take any inspiration from me or continue the story at all, I wanted to express my appriciation of your work. Thank you for an enjoyable read.