they have unrealised feelings towards you.
but you can't just directly awaken them.
you need to gradually build it up inside her and allow her to reach the epiphany herself. then her lewdness towards you will be insatiable.
They instinctually know that that the best way to ensure their survival is to win the favor of a stronger member of their tribe (their oniichan), and achieve this by seducing him.
Trust me, I'm an anthropologist.
It's not even the best incest thing made by mankind.
Only spoonfeeding to promote a criminally overlooked VN.
Imoutos and nee-sans together are the only answer.
Yandere imoutos are a miracle of the universe.