I'm not sure if I'm correct about this, but from what I understand, Bai Ji has the memories and powers of Solanya (which also has some influences in Bai Ji's personality?), who's Yana's older sister, and because of that, Yana treats Bai Ji as her older sister and also has the whole siscon thing for her too (I guess developed from the traumatic experience of having lost Solanya and now having found "her" again?). Bai Ji mentioned before that she isn't, or at least, doesn't see herself as Solanya, but as Bai Ji. She still treats Yana as her younger sister and also made comments about when they were both kids, as child Solanya and child Yana. So my current interpretation is that in the present, they're more like step-sisters rather than actual blood related sisters, since she's not actually Solanya, but Bai Ji?
(Can please someone correct me if I'm wrong?)