Shion Basquiat's truth!

And we finally got the bombshell and, while it didn't resolve the final mystery, it at least closes one of the story threads that has been bugging me for a few days now.
Shion Basquiat was a girl that died long ago that looked almost identical to Mayuri, the screenshot below is a portrait of this girl. Suou and the others investigated in a series of letters they found written by her and turns out she had a relationship with another girl named Kifune Sayuri that was really similar to the Suou/Mayuri dynamics.
The event sequence narrated in the letters was basically identical to the first game of the series except what happened at the end during the festival. In Suou case everything went fine, but in her case the two fought and ended up separated. Kifune regretted her actions (in a final letter that they found together with the others) because then Shion died (unknown reasons as of now) and they never managed to make up.
We don't know precisely how many years ago this happened which makes it hard to understand what kind of relationship she may have had with Mayuri. Was she a grandmother or aunt of Mayuri? There has to be some blood connection as the visual similarity is way too precise for pure coincidence.

I remember Mayuri said that she was raised mostly by a house helper (and she fell in love with her), but I can't remember precisely about the rest of her family structure. How is Dalia connected to all this because she technically share the same surname?