I'd say that Momiji still may not necessarily be thinking about whether her feelings are romantic in nature. She's always simply desired Mahiro's attention, going so far as to get openly jealous when others fawn over Mahiro and whatnot. She very likely doesn't even know why she's doing it; teens do go through a phase like that, after all.
Maybe once she reaches high school (it shouldn't be much longer now, eh?) she'll start to become more self-aware. Based on the material so far though, I don't think it's gonna play out quite the way that one doujin author imagines lol