We need to understand everything Yotsuba knows so far. For example, shes aware that the sister is f*cked up, but for some reason shes not aware of why. I feel like she would have naturally stumbled upon the fact that his sister was messaging the mastermind, yet she remains completley unaware of that.
Also, the mastermind themselves. How can you hate a child this much?? They started before his sister was even in high school, which means they've wanted Uichi dead since he was a kid. And it cant be something like "oh its just fun to torment him" because of their intelligence. They are scarily smart, so even if they wanted to torment him out of fun, they wouldnt because that brings too much risk to themselves, even with all the steps taken for their own anomity. Either we have a complete psycho who got a messed up view of the world fron a young age because of their intelligence, or Uichi did something that so bad in their eyes that its irredeemable.