I think Aya mistaking her for being "oni-san's boyfriend" would be unlikely. If it were true, then the moments when Arai Sumiko puts oni-san and mitsuki side-by-side, and Aya getting this "she looks/feels just like oni-san moments" would be for nothing; it wouldn't be also in-line with how Aya was perplexed/frozen in place in Chap17. Also, I think the author is aiming for a "Oh no, I'm a girl and I like a girl" rather than a "the person I like has a gf" crisis/plot.
