The middle brother is said to be Shukichi Haneda (Sera seemed to be in touch with Haneda in the case in ramen shop, the 'mama' also watches Haneda's competition in TV and so does Akai). Shuichi Akai is rumored to be the older brother (thatSera believed to be dead)i. Sera said that she and the oldest have their mom's looks which is why their eyes are very similar, while the middle brother looks more like their father.
I doubt it's Gin, he killed Akai's girlfriend and they never really met before. they first saw each other from afar through rifle scopes when Gin wanted to kill Mouri because he found Conans tracker on Kir's clothes.
I don't think it's Sukichi either, he is not as mysterious as Akai or Sera, he seems pretty ordinary. he has a ordinary girlfriend and I doubt he would go after her like he does if he was Sera's + Akai's brother who are always in hidding because of danger. Sera might have known Sukichi when she was little. in japan, little girls seem to call older guys oniichan, althougt they are not really brothers.
their whole family is confusing me with Conan mentioing that that little girl looks like Ai, but Akai dated her sister.
Regarding the little girl and Ai thing; Aoyama Gosho stated in an interview that two mothers in the series are sisters. Its clarified that its not Yukiko and Kaito Kid's mom, but when asked if its the little girl (Sera's mom) and Ai's mom, Gosho kind of avoided answering. So that might be the case (Ai is the mysterious girl's niece). But knowing Aoyama Gosho, he has a lot of tricks under his sleeves, so who knows?
It's there any chance that Gin is Akai and Sera's brother?
The little girl seems now to be the mother, so maybe Gin is the middle brother that looks like her..
I have no clue, I need more chapters.