Why there is misunderstanding
This story seems to be taking part before they met or before Shouhei and Mito moved in together
This is kind of a prequel
In this timeline - manga chapter: Shouhei is still living with his parents and brother, so officially - Shizuma haven’t met Mito as the ‘owner’ where Shouhei works.
The parents and brother met Mito when Shouhei was about to live together with Mito :)
The timeline or my clue was Shouhei is still in the house. :)
Hope that clears things up.
Didnt they meet already, nii san and shizuma http://www.mangago.zone/read-manga/secret_xxx/sa/sa-c004.5/3/