Your sister is sad to call an end to the two of you, good thing you have the perfect solution to this problem. “It’s done”, she says. “I’ll still love you, we just can’t keep fucking. I want to get married and be a mother at some point and as much as I’d love to do that with you, we just can’” You have the perfect solution, during your “one last fuck” what harm could come from sliding the condom off. She doesn’t even thing twice, just commenting on how good it feels “are these ultra thins?” After, as she’s sitting, your cum leaking out of her she realizes what happed. What’s a little trap between siblings? Besides, now you can stay together forever