Ever since Sera Masumi was introduced in the manga a few years ago, more and more mysteries and questions regarding Akai Shūichi’s family have surfaced, to the point that the situation around said family has become fairly convoluted and a bit difficult for some readers to follow.
In a series of posts, we’ll explain the various mysteries surrounding the Akai family, summarize what we know, and talk about the popular theories regarding these mysteries.

We won’t be spoiling any cases, but just refer to the relevant information regarding each mystery.
If you are not up to date with the manga and do not wish to be spoiled, do not enter this post.

Disclaimer: The theories that will be featured in these posts are currently the most popular theories in the fandom regarding these mysteries. These are not guaranteed to be 100% correct when the mysteries are resolved in the manga. Moreover just because these theories are popular doesn’t mean you have to subscribe to them.

The first mystery we shall tackle: Sera Masumi’s second brother.

The existence of this second brother (from here on we shall refer to him as “mid bro”) was first revealed in file 859 after Conan questioned Masumi regarding the person on the phone she spoke with, whom she referred to as “brother”. Masumi claims that unlike her eldest brother and herself, this mid bro resembles their father. Later on, we discover that mid bro is also capable of making deductions and has solved mysteries when he was in high school (he also supposedly helped Masumi over the phone in several cases). Furthermore, each one of the three siblings has a different surname (mid bro changed his surname after high school).
After Haneda Shūkichi’s Shogi match for which he receives his seventh title, we see both the “sister from outside the domain” and Okiya learn of Shūkichi’s accomplishment.
In file 928 we also learn that mid bro looks like Yumi’s boyfriend and even shares the same catchphrase.
Finally, in the recent case with Shūkichi and Yumi (945-947), we see in Shūkichi’s marriage registration that his father’s surname is Akai.

The prevailing theory is that mid bro is indeed Haneda Shūkichi, and the marriage registration seems to confirm that. As for why Shūkichi has a different surname, it’s possible that he was adopted as an adult (a common practice in Japan) by the Haneda family, but nothing has been confirmed yet on that front. We’ll get back to the Haneda family in a later post.
It’s important to note, however, that even if Shūkichi is the mid bro, it’s still not 100% certain that the person who helped Masumi on the phone is truly him. (It could have been someone else, but Masumi lies and says it’s her second brother).