Huh? Underage? Who? Huh?

SPOILERS*
So basically it's her older sister that died and Shinju was forced to live as her so no one would find out, that's why her mother never took her out or got her teeth fixed because they could have found out. That's also why they thought she was stupid and did bad at school but she was just much younger, that's why her IQ jumped because she was growing up.