Honestly whether or not it was really cheating doesn't deny how Reiichi is being a total ass during this time, he lied to Kaoru and he isn't making any large effort to empathize with her either when he makes plans or forgets their anniversary and doesn't try to make up for it.
It's a shame because he seemed like a cool chill guy at the beginning and he's Uta's brother so I had high hopes that the plot would be a little more nuanced and not just throw characters into the irredeemable zone all cliché (if it does turn out to be cheating), especially with the hint dropping that he might know of Uta's feelings.
However if it's not cheating I just personally can't forsee any smooth way to bring Kaoru and Uta closer together from this. I mean if Reiichi's any rational human dude he'd rush his wife who fell down 20+ stairs straight into the pavement behind him directly to the hospital and they'd /have/ to make up and have the misunderstanding explained right then and there, meanwhile Uta not being present won't have any good chance to interact.
But ehh that's too much overthinking. I still prefer this over NTR which I gave up since the moment it went into that cheating shit and Citrus became a bit stale now no offense. The art for this is well above average too and if it's recommended by Nakatani and was very good so far I still have good faith that the author can surprise us and make this work out.