It's at least partially the younger sister's fault though. Anyone with half a brain that was close enough to them would've noticed the older sister is way too into the brother by now. Yet, the younger sister sees this and does what? Treats the brother like garbage. Of course he'll treat her in kind, and of course the super dreadnought class brocon will become hostile to someone that mistreats the brother.
She wouldn't be any better off if she was TOO kind to the brother though, because the older sister would act the way she did with that store manager. The best path forward would've been for her to just act like a normal sibling toward both of the other two, but she is herself clearly something of a siscon so that's exacerbating everything because the older sister doesn't want the attention and the younger sisters jealousy towards the brother for the attention he's receiving probably makes her even harsher towards him.
TL;DR: Acts like a bitch, treated like a bitch, acts like even more of a bitch, rinse, repeat