It appears that Yotsuha was born from an affair between Fuu-chan's dad (Fuu is most likely named Futaba, following the naming scheme of Yotsuha and her brother, but with the surname Futatsugi that leads to her name having 2+2 and being 4 just like Yotsuha) and Yotsuha's cult leader mother Kazuha (her name means either one leaf or number-leaf so she's also in the naming scheme). Fuu-chan's father committed suicide (which is why Fuu-chan has this big reaction in this chapter) right after Yotsuha was taken in by her mother. The news announcement about this can be seen in chapter 40, after which Yotsuha's mother says "I guess I don't need him anymore".
My guess is that Yotsuha's mother decided to take her in in order to blackmail Fuu's father, or as part of a continuous torture campaign akin to what Uichi was going through (Kazuha's shtick is that people are more lovable the more they suffer, because it gives them depth), but because he died she decided to just have fun with her. The first year of middle school, when Yotsuha becomes sick and the mother no longer has access to her and "dies" in a car accident (the circumstances make it unclear if she's really dead), is when the mastermind approaches Rei and Uichi's torture campaign begins.
In the flashbacks when the mastermind talked to Rei, they said they want to get revenge on Uichi, because he killed [censor bar]. Given the timeframe, it basically only makes sense for it to be Yotsuha (with Uichi being used as a proxy) being blamed for killing Fuu-chan's father, and the point of the revenge is for Yotsuha to feel the same thing Fuu (and/or her mother) felt seeing the dad's hanging body. An important point is that the dad, Uichi, and Rei all used the same suicide method.
In chapter 43, Yotsuha mentions having been in contact with Fuu-chan after starting high school and would have most likely told her she was going to go visit Uichi, allowing Fuu-chan to know the timing to have Yotsuha find out (and feel responsible for his death) and be the one to tell Yotsuha about it afterwards.