Is this so obvious that I always forget Taehyuk's name-
chapter 56 will give you the answers.
im guessing it's the sister. i dont remember much but i think only 2 pwople survived from the slaughter the sister and the MC.
remember how in the present time the family believed that the dragon's mate was a female?
maybe it's because he just blessed the sister of the past, hoping his mate would be reincarnated into her family tree (rather than blessing the other blood relative who's obsessed with his mate and also indirectly killed him)
that and also due to him blessing the sister maybe information/history kinda got blurred all thru out the years making the family think it was female
I wonder if we're gonna see something like mass murder or slaughter etc. And how did Hyo-un become the dragon of that family and how did he know that his bride will be incarnated into that family