I think Sera is Akai's sister, or maybe a MAFIA's person, because Mr. James knew her, so that can only be 2 reasons:
1. Sera is Akai's sister, Mr. James might have met her in Akai's profile when he attended to be a FBI. (Mr. James is the boss, so he might have kept his profile)
2, Sera is a MAFIA's person, Mr. James had met her when Akai was in MAFIA, she might be a close friend to Haibara's sister when she was in MAFIA, so Akai might have told Mr. James about this person (for prevention)
3. Sera can't be Eisuke or Eisuke's sister, because Mr. James didn't know Eisuke very much, so when he first saw Sera, he couldn't recognize him immediately. (Also, he didn't know his sister, how could he know she's Eisuke's sister at the first sight. Impossible!

)
So, in conclusion, I think Sera can only be Akai's sister or MAFIA's person
THE END