It’s bc the previous violence from their dad gave her trauma of probably men who are larger than her due to the repeated abuse so yes she’d flinch at someone of a similar stature even if it was her own brother sadly, and especially bc she witnessed youngchan in a violent manner (not inflicted towards her tho ofc)
idk it makes sense but again it doesnt to me like, what was he supposed to do? Idk maybe certain details being added would make me feel better about that part of the plot lmao like I wish an added conversation between them where the end result would give him closure about it (maybe there was and I forgot cuz it's been a while since I reread it and i havent finished this current reread lol) because that part of the story always felt lowkey unfair towards youngchan. I just hated how he felt like he was just this scary person who would be hated if someone saw a certain part of him because of that entire situation when he was just doing what anyone would do. I get when he came up behind her and she kinda jumped cuz it surprised her but it seem implied that she was actively scared of him after protecting her(to me, idk I could be interpreting that wrong) that part always seemed weird to me when I read it cuz I get if he just was senselessly violent but he only did what he did to protect her and himself. Also it's probably just a stupid detail to even be hung up on, I def acknowledge itttttt idk why in the entire story that's the one thing that always bothered me lolllll.
One thing I have never understood when reading this was why youngchan was treated like some scary monster when he defended himself and his sister lol. Your dad just slapped the fuck out of you, then he was about to hit your brother with a golf club so your brother knocked tf out of him in self defense and you're scared of your brother???? WHat????? AM I MISSING SOMETHING????? He was the one who slapped the taste out of your mouth and youre scared of MEEEE HUHHHHH????