If Shukichi is the middle brother, then maybe he heard about Conan from Akai (Like Haibara learned of Conan from Akemi). Because it seemed like he trusted Conan's detective skills, when they met for the first time. I have posted before a theory about Akai and the middle brother being twins, and if Shukichi is the middle Brother, then I think it's very likely that they could be twins, as they both share a kanji for Shu in their given names, and ichi in Shuichi means 'one'. It could mean that Akai was the oldest.