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.