Sera saw her dead brother appearing in front of her on the train. I assume Akai wanted to clear things up so she won't search for that persona (scar Akai) and won't look into it. Not telling her would be cruel and dangerous.
Shouldn't the same apply to Jodie and Camel?
+Additional Inquiry: If indeed Akai is the brother Sera is talking about, why did he ask her, in his e-mail, if she met the magician? He saw them both together as Okiya (during the Kudo Yusaku's Cold Case) and his question thus is pointless because he knows the answer.
Please note that I am just analyzing, and not seeking to prove my point as I do not have the full answer to this mystery yet ==> So I will raise another troublesome issue that might favor Akai being the only brother (in spite of me doubting it).
According to the manga, Okiya is 27 years old, so is Shuukichi. The twin theory will not work based on the older/middle description Sera mentioned. I don't know if we can safely assume that Akai is 27 based on the age assigned to Okiya (Since he might be posing as a younger person, and not only as a different person). However, according to a recent interview, Gosho Aoyama-sensei did not want to explicitly give Akai's age. Could it be that this has something to do with our mystery?
If Akai is indeed 27, Shuukichi cannot be the middle brother. We will be left with the other possibilities in the Poll.