Mimi,
Was your cousin fucking the guys while you were actively going out with them or did she wait until you were done with them? Was she ever the cause of one of your relationships failing?
What is your opinion here? Were you two really tight when you were growing up? Is she involved in some amped up but misplaced 'sibling rivalry' type thing where she feels like she's competing with you to be the most attractive and sexy of the young women in the family?
Since it sounds like she's active with this guy right now, do either of you have any interest in this guy beyond his big cock? Did she tell you about him and brag about his cock size like it was some prize she won at bingo and expected you to be jealous of?
You've been with this guy once. Did you tell him you're the cousin of this other girl he's fucking? How did he take the news if you did?
If neither of you are emotionally attached, have you considered a threesome? Maybe just the proposal of having one will drive home how her poaching your cock made you feel.
