What are the familial relationships in Gentlemen's Alliance Cross?
Haine was born into the Kamiya family, but "sold" to the Otomiya family in exchange for 50 million yen, hence why her family name is Otomiya.

Shizumasa is the emperor, but we first meet the "fake" Shizumasa, who is filling in for the real one, and we find out the fake is actually Shizumasa's twin brother.

For more details including family tree diagrams, see the GAC wikia:


Haine page
Kamiya family page
Togu family page