Seems like we entered the egg villainy and slander arc...
Well they have been dating for a while now, so it is more of a re affirmation
Pretty sure they weren't dating as Chapter 55 is the first genuine romantic interaction they've had.. in-fact literally their
first ever romantic experience for both of them. Remember how Sumireko immediately regret wanting to enjoy disguising as a schoolgirl when Oto's friends started asking her about intimate experiences?
Not to say that Sumireko wasn't already aware of Ren being romantically interested in her before then.
You're probably mistakening when Oto first asked if Sumireko was her brother's girlfriend in Chapter 5, before Ren affirmed they were just coworkers.