So Basically she's been a psycho since she was a child. I'm not saying there's not a reason behind it, but seriously, she burn animals and people. That ain't good for a kid's mental health.
Are they showing this story because one (or both) of them are gonna die? I hope not.
"Hey step bro, it is cold, do you want me to give you a little warmth?" (Proceeds to put him aflame)
Holy shit I thought this was some kind of simp thing and he was in love with her despite her being batshit insane and treating him shitty. Turns out she was just his fucking sister all along and nobody knows, now that was a twist I never would have guessed.