"i love my sister and would do anything for her, but the guy she's fake-married to that she's liked for years - and whom i know likes her back and is trying to win her over - is looking kinda cute. i know! I'll deliberately try to sabotage their marriage by giving him advice that would make him come off as creepy; then I'll tell him that he scared her off, and try to seduce him away from her in their own house, so that she can come in and see me on top of him - breaking her heart and making him think that she hates him, so i can then try and manipulate him into being with me instead!
it doesn't matter that it'll make my sister hate my guts if it weren't for the trope of characters forgiving the most heinous shit just because the author is too lazy to portray the resulting dynamic if they didn't and that her best friend just tried to pull basically the exact same thing as me, and that the double whammy from being backstabbed by both her sister and her best friend since childhood (and the guy she likes, if i succeed) will give her trust issues for the rest of her life if the author were to actually write her reaction to it in a realistic manner, and that if i do succeed then the situation she'll be left in is exactly the sort of stuff you'd expect to hear from someone looking to put an end to it all not that she'd do that, because that'll raise the age rating. but hey! at least I'll maybe get a cute guy to reluctantly settle for me for the rest of his life! totally worth it!"