Yes, there will be a love interest (well, two, I guess) that is NOT the sister. The sister is very, very much the antagonist, and the MC absolutely HATES her. So no, no incest here, biological or otherwise.
i would commit suicide with no hesitation in his situation lol.
That girl is no longer a yandere, she is a worse than a demon. I can already see the whole family massacrated unless mc manage to kill the sister... but given that its a world of magic, she may survive even if she is decapitated...