Sacchan is growing up in complicated family circumstances. Her dad left when she was 5, they're poor and live in a tiny apartment. It's also dirty because her mom doesn't have time to clean. Her clothes are old, often second-hand. Her mom takes out her frustration with her own failed life on Sacchan pretty much every day, but Sacchan is of course too young to understand that and thinks it's because she's doing something wrong.
Despite all that, she tries to be cheerful all the time and keeps her feelings bottled up deep inside. She probably keeps telling herself that she just doesn't want to bother Yui and Kotoha with her problems, but the truth is she's just scared to talk about those things because she realizes it's not normal and doesn't know how they would react.
I've known girls like her. She needs to be very careful, as girls like her are extremely susceptible to emotional manipulation. Any guy who pretends to care about her and gives her attention could easily get her to do things she can never take back. Her mother doesn't pay attention to Sacchan at all, so most likely she wouldn't notice (or notice when it's too late).
That said, I wish I could talk to her and give her the support she needs. She might seem cheerful to most people, but I can tell she's just a fragile little girl.