Soleil, Ophelia, and Caeldori (who is, in my headcanon, the daughter of Selena) often saw each other growing up and consider themselves as cousins.
Also, in my headcanon, Selkie is the daughter of Hinoka, and part of the reason she's so reckless (jumps off a cliff in her C-Support with her mother, thinks of battle as a "game") is because she doesn't entirely understand how Dragon Vein works and thinks she could use it to protect herself if things went REALLY south.