So I tried to think of characters we haven't really incest'd before for this prompt.
>your older sister Starlight Glimmer ran away when your were little.
>you don't have many memories of her, but your father always held out hope that she'd return.
>and she did, years later when you were in college.
>turns out she started some kinda weird communist cult and wants you to join.
>you learned she doesn't take no for an answer when she had some of her minions kidnap you in the middle of the night.
>apparently in her (likely drug-induced) madness she thinks she can see the future and 'prophesizes' that if you impregnate her, she will give birth to the savior of humanity.