Nagisa is either adopted in Kiryuu family or, more probably, his mother was from Shinamiya family.
While he is still share similar look with his brothers, they don't think of him high. Also they have some unexplained hatred towards other founding family of Kiryuu industries.- Confirmed: in ep.6. Nagisa was born from Yasuhisa Kiryuu's affair with Chisato Shinamiya.
Iris is Nagisa's sister
It's hinted that they know each other for a long time. Also she lost her memory about ten years ago (probably in the same incident there Nagisa got his scars). So she is either his childhood friend or sister. The second guess will be more convinient for the plot cause this can make a decent Morality pet for Nagisa and cause author already ship him with Mizuki- Hinted in ep. 6: Chisato was the only relative of Renichiro Shinamiya and in her nightmare Iris calls the man, who could be Renichiro, "grandfather", so she is most probably Nagisa's half-sister.
- If they were related, she would probably have been adopted into the Kiryuu family too, as she would also have a valid claim to her family's shares. Right now it looks more like she belongs to a bodyguard/servant family.
- Jossed. Nagisa is a stand-in for the real heir, Iris.
Subaru is actually female.
In episode five, he was mysteriously absent from all the beach scenes, and declined to go into the boys bath while covering his chest. Iris and Hanako are related.
Seems to be more likely right now than the other relationship WMG. Taking the different surnames into account, it seems like they are cousins or so.