I love it when new characters are introduced, and (my memory might be faulty) the last major new character to be introduced is Borboun.
Therefore, reasoning why this unknown brother may know of who Conan is, is because Shuichi Akai had consulted with the second oldest sibling.
In other words, Shuichi could have talked to his younger brother about Conan, in the event that something bad did happen to himself. With that being said, it could also be possible that Shuichi's brother is also in the Black Organization to spy on them for the FBI. In which case, this goes back to the theory that the middle sibling could be Amuro/Borboun.
The middle brother is Shukichi Haneda
My post here