I've noticed throughout this flashback and just before, she sounds very focused on the idea of replacing her sister and an unhealthy fear of death.
It doesn't directly support my theory that she's a clone of Shine, but its very telling that she was the only family member in her life.
Besides the flashback with her grandfather, we don't really see any memories of what she thought of her other family members, ie her parents. I'm not saying they don't exist, but the fact that they barely played any role in either of their daughters lives feels almost deliberate.
Was the fact that their older daughter being born without emotions and limitless power, something that happened by chance? After introducing her to their younger daughter, why was she practically the one to raise her? Throughout the entire time she had been living on her own, has Shy even gotten any form of communication from them or her grandfather?!