This is my hot take but
Holy shit his sister is an utterly selfish creature.
She refuses to compromise or even take things slow, it's all about satisfying her lust, her way right now or she's killing herself.
He's only doing this because she's suicide baiting him into this and at every point she ignores his feelings and comfort unless it's what she wants them to be.
If you don't care about your partner's feelings and demand they only feel what you want them to feel, that's not love.
"B-but incest is wincest!" does not excuse her being terrible.
Agreed, I feel like half the (dark) comedy is how malignant her mindset is. She doesn't actually value him at all beyond the idea of what he
should represent to her. She's an absolutely repugnant little girl, but it's not like it's uncommon for kids to be lacking in empathy or introspection. She needs a fuckin parent. Someone outside of her brother, who is also a (slightly older) child and clearly waaaay out of his depth.
She will not be given this parental figure however, because that would be a very different story. I actually think it would be pretty fun to read a story about the recovery and lifepath of a girl who permanently tore up and obliterated her familial relations under the weight of a misguided and one-sided incestuous attraction. Fr like... does anybody have a rec? I want to read this.