As for me, I think that Sera is actually Akai's younger sister, or to be clearer, Sera looks up to Akai as an older brother or he is an older brother figure to her. It can be that she and Akai may be blood-related or not. But I have HUGE questions popping in my head right now.
Okay...back to Sera. MAYBE she is Bourbon, and she joined the Organization in order to find out more about Akai, or investigate his death, and maybe she knew that Akai had some sort of connection to the B.O., and tried to get into the B.O. in order to find out more about Akai and or his death.
That's all I've got. Let's just wait for EVERYTHING to unfold!
This is so exciting! I can't wait to find out the answers to these questions!