Boyfriend Helps Family Member with Incestous wish
I had a weird little idea for a Thanksgiving story which I'll share below but since I love hearing other writers' ideas I'll just say the general idea first:
A boyfriend/spouse/fiance is spending time with his girl's family when he discovers that one or more of them is attracted to his girlfriend. While the family member is quick to tell the boyfriend that he/she would never act on it, the boyfriend actually aids them in approaching his girlfriend about it provided that they keep two rules in mind:
1. It has to be consensual
2. He gets to watch/participate
The idea I had was a bit convoluted but I wanted to share it so I could get some feedback. The boyfriend in this case gets invited to go to his girlfriend's (who is more of a fuckbuddy) house and have thanksgiving dinner with her family and spend the weekend before taking her back to college. On the ride there, his girlfriend teases him about how she won't be available to him the whole weekend due to dinner preparations/shopping/visiting the family. The boyfriend then jokes that he'll just have to seduce her 18-year-old sister and the girlfriend just laughs, saying her bookish sister doesn't show an interest in anyone.
As they get closer to the girlfriend's house, she decides to place a wager. While she doesn't want to see her little sister get hurt, she does want to humble her boyfriend a little. If he can get her sister to make out with him on Thanksgiving or Black Friday, the girlfriend promises to stay home on Saturday and let the boyfriend use her however he wants.
The boyfriend takes the wager and immediately starts trying to get closer to his girlfriend's sister. The girl likes the attention but still tries to keep the boyfriend at arms length throughout Thanksgiving, which makes the girlfriend amused to no end. The boyfriend's about to give up when he walks past the sister's room, where he hears her frantically typing. He waits until she slips out of her room and finds out she was working on some erotica. He takes a minute to find her username and then notes that the sister seems to focus on one genre specifically: Incest. More specifically, incest between sisters. Most intriguing of all, one of the sisters always looks like his girlfriend.
The boyfriend waits for the sister to get back and confronts her about her writing, before telling her about the wager and making her an offer she can't possibly resist.