In the second part of the series after the first time skip, Saki learns that she did have a sister called Yoshimi. According to the comments made by her mother, Yoshimi was killed by an Impure Cat.
Yoshimi showed signs of having gained her powers early but didn't seem controlled because of how her name looked in the mirror Saki found. Considering that she didn't look all that much older to Saki,
Was Yoshimi really that much of a threat to have developed her powers so early that she had to die?
Isn't the purpose of the Unified Class to help children and teenagers control their powers?
Why couldn't Yoshimi just be a child prodigy who could one day take Shisei Kaburagi place?