
It's happening at almost the same time, you can see that scene in bar when they met in the prequel. Minato didn't participate in that dinner

ok after seeing the raws i think i get it now. the first couple is already together, however, i don't think theyre living together yet... maybe this takes place between the two volumes for the first couple? anyways one thing is for sure - shouhei knows mito and minato are brothers. judging from the raws, mito does not know shouhei's brother yet and minato does not know the two are related. obviously, shizuma does not know mito.

They met in chapter 2 of Secret XXX, and Shouhei introduced Mito around chapter 4. But right now Shizuma clearly doesn't recognize Mito, so it has to be before that

Ok but where are the raws ??? please

But then, when Minato was distracting Shizuma's and Shouhei's mother from what was going on between Shouhei and Mito (in chapter 4 of Secret XXX), little did she know that Minato himself was sleeping / in a relationship (depending on how the timeline works in the end) with her eldest son.
This series takes the concept of quiproquo to an extreme (⊙…⊙ )
Complicated. Shouhei knows that Mito and Minato are brothers, but they don’t know that Shizuma and Shouhei are brothers. Same goes for Shizuma. I wonder if Minato and Shizuma worked things out before Minato met and share a room with his mother. Mito had dinner with Shouhei’s family, was Minato there too? Just coming up with things to think about so I won’t be depressed after seeing chapter 4.