If by chance, I were him, I would "pragmatically" kill my sister beforehand since there wouldn't be any good by keeping her alive (well, this is a radical thought, so I know this is bad, but the voice inside me says that she should die, and I believe that there are people thinking so). I mean really, there are two possibilities, um no, three possibilities that can happen here. First, you can imprison her (most favorable, but we don't know what will happen to the sister, will she be good or still bad?; But ofc, this is the best solution, though the sister was too clever and cunning that the MC couldn't even imprison her even if he is smart since he was still outsmarted. I believe that if I try to use some trick to imprison her, I'll be ridiculed by her instead since I'll be most likely to lose the game). Second, you can't imprison her (you'll end up like the MC, always be in disadvantage and used). And the last, you can kill her (the extreme possibility one). If you ask me which one I would prefer between the second or the third possibility. I would like to choose the latter. I mean, yeah, the possibility of me getting arrested is high, but that doesn't mean I can't escape (I know this is bad, but think it carefully, the sister can escape from those crimes, so why can't I?), and even if I get arrested, it'll still be better. The fact that she dies won't change.
Well, this is just my wild thought. Personally, I don't really like this nor recommend anyone to do this. I believe there won't be many people (or maybe there won't any people) that agree with this kinda train of thought. I'm sorry if my comment is too disturbing, peace for y'all.