You know, something that will probably be missed but I can't help but bring up:
When Yuu sees the picture of Touko's family, she sees her sister (of course), although we, as readers, have never actually seen her before.
We are made to assume that, in her journey to become her sister, Touko took on her sister's appearance, as we knew that she had long hair, and the young Touko's hair only went down to her shoulders. Basically, a physical reflection of how she was always trying to intimate what her sister was.
But in that moment, Yuu mentions that Touko takes from their father, while Mio took from their mother. "You think so too? I get that a lot."
The two were never the same from the start. No matter how much Touko tried, she never was her sister, even while she tried to embody the idea she held of her sister's "perfection", and not even the people who knew her sister thought she was.