On page 20, Umi gives the classic "do you want dinner, a bath, or how about me?" line, which is pretty much the most stereotypical welcome-home-honey wifely remark possible in Japanese. That line, all by itself, is more than enough to make clear that they are a couple, and one in a cozily domestic type of relationship.
That very stereotypicality makes it prime joke material, though. Conversely few pages later Kaede muses to the effect that she mainly rebuffed the Bosslady due to wanting to keep work and private life separate - all the while screwing Umi silly no less...
Either she's an impressively terrible person or the two of them are more "friends with benefits" than a couple per se.
I read the "keeping work life and private life separate" part to mean that she would be up for it if she wasn't in a relationship with Umi...
Or if the lady trying to seduce her wasn't her boss? I think Kaede's acting too chill to be in a completely committed relationship with Umi, but then again, I have a lot of friends in open relationships and I never assume people are actually dating unless straight up confirmed. Too much trauma from ships being sunk, unmutual love, etc.