Sumi was going go have a story the mangaka ran out of pages though....
You would know if you read the last part from the mangaka
Going to*
. , you would know if you read the last part of the manga that the author considered using the Extra area for the older brother, but there wasn't room. And in that Extra, the author said that it was both likely that there would **and would not** be a story just for the older brother, and if there was and we read it, to think kindly of the author. In other words, the author would like to, but may never get around to it or may not be given the opportunity to tell his story.
I'm with just a passerby and wishing the older brother gets his own tale.
I wish the other one-shots in the beginning had more development than just being one-shots. They seemed really interesting. Plus, I wish Sumi had a story. He seemed like a character with a lot to share.