I don't know where the author takes this one, but it's quite common for kids who've been thrown away to reconnect with their "real" family members later in life, and lean into it hard, sometimes at the expense of their adoptive family ties. It's to fill a void left behind by the act of abandonment.
Also, in my impression, Japanese often do their family units in binary. You either are, or you're not part of something, there's not a whole lot of nuance and lengthy descriptions, just less or more defined roles in the family. In a sense, this means that the other sisters switch from "are" to "were", with a huge change in what their relationship means to eachother.