Pei Ye had in fact more or less realized it: since the woman and Sisi were clearly not biological sisters, she must have been adopted by Chancellor Xu later.
"I don't know whether you had a lonely, helpless childhood, though with Yue Muzhou beside you, perhaps it was a little better." This subject seemed to silence her even more than "Sister Sisi," perhaps because it had lain buried in her heart longer. But now she spoke earnestly. "From the time I first learned to walk, I never received any kindness from those around me. Everything in my memories was dark and thorned... the only little glimmer of light was that, back then, there was always a woman keeping me company."
"..."
"She was very beautiful, with the gentlest smile. When no one else was around, she would quietly appear and take me to catch dragonflies, weave flower wreaths, pick fruit... but what I loved most, what made me laugh the happiest, was swinging high on a swing." Xu Chuo spoke softly. "Because every time I reached the very top, I could catch a momentary glimpse beyond the walls, of countless tiers of overlapping eaves stretching endlessly like ocean waves. They let me know the world was actually that vast."
"But later, I could never find her again."
"You couldn't find her?"
"No," Xu Chuo said. "I don't know when it began. Perhaps just after I was past that stage of knowing nothing. When a child is too young, there is no notion of 'parting' in their head. All I knew was that I never saw her again, and could find no trace of her. Everything was like a dream."
"..."
"'Swinging Rope' is the story of her that I want to find and then write down-it might sound a little