'I see how you look at my sister.'

(p1/sister betraying sister)

IN WHICH, your sister has a huge crush on Sirius Black. Like most girls at Hogwarts do. Only, Sirius is also crushing on you. See, this is why family dinners got awkward whenever the topic of boys was brought up, in your house at least. Of course, you were a good sister, so you never persued Sirius or said yes whenever he asked you out. But he was starting to wear you down, and eventually you had a bit of a crush on him. So you found a solution. Date him in secret until you have the courage to tell your sister.