
Maybe it is about sleepwalking. They might both have the same ability to travel through other worlds. Like inheriting her mother's powers as the other families do in this world. Because I remember Sanya saying that she had the same face in Korea that she has now. So her body is probably the same in both lives. Her mother also traveled to Korea when she died here. Then had a child there. Something must have triggered Sanya's power that brought her back to her mother's world. It could be dying.
There are holes in this theory I know but probably there are still many things that the author didn't show us yet.
I just think these things that the author showed us are too good to be just coincidences.
Ok here is my theory. We know that Sanya's mother from mordern Korea loved freesias. And the empress's sister also loved freesias. The empress's sister was also sleepwalking and pur heroine also has this problem. The empress and Sanya have also the same eye color. So. Sanya is the daughter of the empress' sister.