I assume their story might be mentioned a bit if they go into the history of the steppe in the lorebook, since it's known that both clans originally lived there, but most if not all of the Raen decided to settle into areas away from the steppe at some point.
Considering that the guide at Sui-no-Sato makes mention of them being ousted from their ancestral homeland and it also being mentioned that they were originally nomads in the custom delivery questline, I feel like it's implied there may have been some kind of ancient long-forgotten dispute between the two clans that led to the Raen being forced out of the steppe.
