Even if her being a psychotic lesbian who hates her brother was justified in some sort of contrived way or just a misunderstanding, I’ve never really seen the appeal of introducing this sort of unrequited sapphic love competitor that serves to do nothing but stir the pot.
She’ll never be a realistic love interest for fmc; she has nothing interesting to say that could affect their current dynamic. Her influence on the story is entirely self-contained within this most recent development.
This arc sucks, and every time an author does something like this it sucks especially extra, because nobody has ever liked it. It’s noticeably bad here because none of the male leads have done anything at all to warrant her spite, so she’s just childish. If anything, it’s even worse that he’s blaming himself for her own issues!