I feel like Yanagin already liked Kase from the start. I mean, the way she talked about her and she planned was kinda suspicious. She was just blind by envy and kinda tsun to recognize.
This is literally "What do you call this trash" I actually liked it and recommend it to anyone who likes this
The plot is the same with the secret spying on siblings and rewards for the work except it's not incest or bl or gl