Hmm, I'm not seeing that at all. Maybe this is going back several years so forgive me and my fading memory, but does Dylan seem like he thinks his sister needs to be "punched in the mouth", as you say? Pissed about the blue pill prank, yes, maybe jealous of her achievements, annoyed by her teasing, but doing something substantial enough/mean enough to cut her down?
How do you envision Dylan bringing Ellie down to earth? We expect there's no more blackmail being introduced so it's most likely not to do with any sort of threat of exposing her (whether her activities with Julia, Logan or even Sophia). Doing something as outrageous as she did to him with the blue pill prank or somehow sabotaging her school, dance, cheerleading, or her pursuit of either Julia or Logan (or even Sophia)? Any of those would just make Dylan an even less likable character, right?