I kind of wanted to revisit one of my sections of my currently oudated theory about the middle brother, now assuming that brother is Shuukichi and the Wizard is Shinichi. At the time, I already made the assumption that Masumi knows Conan's true identity, and thus can deduce Shinichi disappeared for some reason and Conan appeared in his place. Shuukichi knows Shinichi is Conan, but is keeping this a secret from Masumi This argument has two parts: 1) deducing the unseen contents of Sera's text and Shuukichi's return text main body, and 2) analyzing the exchange to prove Shuukichi may know Conan is Shinichi. It also assumes that Shuukichi knows the Wizard refers to Shinichi. The two unseen text parts were Masumi asking Shuukichi how he knew Conan and Shuukichi's reply that he met Conan at a case prior After the exchange in the bowling alley case (Masumi shows her brother Conan's picture thinking he will be surprised. Masumi is shocked when the brother replies back that "if it's him, then there's no problem".(861.4)), Sera texts something we didn't get to see to her brother. Shuukichi later sends a reply to Masumi that we also don't get to see, which makes Masumi react "Ah, I see. So that's how it is." In that same reply's PS, Shuukichi asks Masumi if she has met the Wizard. Masumi knows Conan didn't exist prior to Shinichi shrinking. That means that her brother must know Conan because either: 1) he some experience with Conan as Conan or someone talked about Conan with him before 2) he figured out Conan is Shinichi. Masumi doesn't yet know which of the two explanations are true for her brother. Given the previous exchange, the single, most logical followup question Sera could ask her brother is how he knows him. In fact, that's exactly what Masumi was thinking after she got his message (861p4). Masumi's thought reply "Ah, I see. So that's how it is."(861.15-16) to her brother's answer fits a conversation where Shuukichi explained something after being questioned. The reason why this part of the conversation was kept hidden from the reader is because it would spoil who the middle brother is because it would shortlist Shuukichi as a suspect who had met Conan during a case.
Circling back to which of the two possible explanations above applies to Shuukichi, the one he must have claimed in the reply can only be the first; Shuukichi had a prior experience with Conan as Conan. Specifically Shuukichi said he had been with Conan when a murder case happened. This is the most likely response because it is truthful. The only reason for Shuukichi to use another explanation and lie is to avoid Masumi pressing Conan for details on the murder that could lead Masumi to his doorstep. I don't think that is reasonable because Shuukichi doesn't seem especially concerned about avoiding Masumi. He did due diligence in not telling her where he lives or that he is a professional Shogi player, but he isn't avoiding public appearances where she might recognize him.
One thing Shuukichi's reply could definitely not be is something like, "I recognize Conan is Shinichi" or "I met him a long time ago". Sera's reaction to his reply was some surprise, but not a whole lot, as well as a little happy blushing. Overall, his answer satisfies her. If Shuukichi had tried to cover for himself with a vague half-truth like "I met Conan a long time ago", Masumi would catch the lie and not be satisfied with that answer because she knows that Shinichi only disappeared recently. If Shuukichi had come out with something like "I know Conan is Shinichi", that would have surprised Sera more than she was shown to be. Also, that would mean Sera would now know Conan is Shinichi and there would be no reason for Shuukichi to then ask if she had met the Wizard (Shinichi). Shuukichi may have brought up the Wizard as a test for Sera because he knew Conan is Shinichi After Sera texts her brother Conan's picture without telling him about her suspicions about Conan's true identity, Shuukichi then asks if she met the Wizard. It seems non-coincidental for Shuukichi to bring up the Wizard right after Sera reveals to him she met Conan. On top of his high deduction skills, Shuukichi has a special ability "the best memory in Japan" that would enable him to figure out Conan was Shinichi decisively. We know he has a memory for people because both Yumi and Masumi attempt to leverage his ability to remember details about suspects for a murder case. Masumi has neglected to tell her brother that she knows Conan is Shinichi, and until now she had withheld the identity of the detective she had been competing and collaborating with, so the brother is probably not aware that she knows Conan's secret identity. Thus, he may have asked her about the Wizard in order to probe if she knows Conan's identity without revealing that he himself already knows Conan is Shinichi. At this point, the main issue is when Shuukichi had the opportunity to meet Shinichi so he had a reference point when he met Conan. The middle brother might have been present at the ocean meeting with Sera, Shinichi, and Ran since he knows about "The Wizard" in the first place. The decisive evidence would be Ran's feelings about him. So far Ran has felt Masumi and the unknown child are familiar, most likely in connection with the past waves event that has been foreshadowed. Ran and Shuukichi have not been in the same cases together so far.
The other, less important problem is whether he understands the shrinking problem, after all it is more likely to think two children from the same parents with different ages look exactly alike than one is the same child shrunken. Shuukichi does have two potential sources of info to help him. One is Shuuichi, his brother who he may also advise on cases. (I feel this is unlikely because Shuuichi is very reticent regarding deductions.) The other and more likely possibility is that he is aware of "the unknown sister from outside the domain" who many presume to be shrunk by APTX or something else. They are avoiding each other because the child feels she is under threat and does not want to drag Shuukichi into danger.