The perfect family: Tangled webs of obsession

Idk why scenario doesn't have many fanfics, or I'm just dumb and can't find any lmao. ANYWAY, here's the description
Book II: https://www.wattpad.com/story/352837858-book-ii-trapped-in-a-web-of-lies

A perfect family. That is what Rui was looking for. He wasn't looking for power, only a perfect family of his own. Everyone who became a part of his spider family, always failed. They got killed. No one was worthy.

Until two siblings stumbled on his path. The older brother was a human and a demon slayer, yet the demon sister protected him. That's what he was looking for. The demon girl was his ideal younger sister. Rui offered for the boy to live if he gives his sister over, but the boy only says:

"I will never give my sister over! I will protect her no matter what!"

Protect... It rangs in Rui's ears. Then he realizes something. The perfect siblings he's been looking for isn't just the girl, it's both of them.

"Nii-san, little sister, welcome to the family."