
What are you talking about stupid Ogi, she IS the royal family
http://www.mangago.zone/read-manga/akatsuki_no_yona/an/d_v-27-chapter-156/pg-31/

As far as ruling goes, cousins do not count for anything because in the hierarchy of succession, a cousin is not that high. Royal family in this case is Ruler (Soo-won), spouse (none), children/heirs (none), parents (no father). Anyone else is more or less like a branch family and are only important if the royal family dies off.
Like if i say my family consists of five people, most people will assume I mean parents, siblings, and maybe grandparents. Even though I have aunts and uncles and cousins, they are considered their own family separate from mine.
This is the one yaoi where i wish the top was faithful to the woman. Still hate Akitora for cheating on his wife.