It’s just speculation on my part, but based on the last few chapters, I think the head of the Ji’s family might also be from the parallel dimension. When he arrived here, he may have seen an opportunity to change the course of events by taking power away from the Gye family early on.
I love the story, but i kinda lost in it, like what was the reason the Ji's having so much beef with the gye? Has it already been explained?