I thought too. Wondering if she loves it bro or is it a twisted love. Or just a caring sister love. I do think that she s knows more than she's letting on about.
Yeah it could also just be that she found out that her brother and "boyfriend" were a "thing" and she wanted him to say it outloud and when he didnt say anything to her she just flipped
I lind of think that the sister likes her brother which is why she is reacting the way she is....... its just a thought though i could be 100% wrong