But Ichika doesn't know that - to her, all of this is very abrupt, and questionable.
Like, for one, why would that make what was going on here make less sense? She's a teenager who realized the feelings she obviously had already and immediately went to tell the girl she likes about them. That's extremely normal. It wasn't Ichika who initiated this and confessed. And her confessing cleans things up with Ichika.
For another, Ichika barely knows about the stuff between Kawase and her sister and had just overheard Kawase and Shiori talking about Kawase's feelings for Ichika. In particular, this exchange was very telling:
Shiori: Who... are you thinking about right now? It's not Rei, is it? When you stopped me back then, that meant you didn't want Ichika-chan to know, right?
Kawase: Do you... think Mori-san is cute?
Shiori: Huh? I'd say she's average. I'm way cuter than her.
Kawase: Don't you think about wanting to be with her?
Shiori: No.
Kawase: Don't you want to touch her when you see her?
Shiori: Not at all.
Kawase: Don't you find yourself constantly thinking about...
Shiori: Pretty much never.
Then Shiori stopped her from saying aloud that she's in love with Ichika which she had obviously started to say to tell her that she should tell that to Ichika's face because, obviously, Ichika could hear all of that. If this was a surprise to Ichika after that, then she's denser than she's ever been presented as before now.
Ichika has also spent awhile now seeing Kawase act obviously into her and has acted upon the idea that Kawase is at least potentially in love with her before because, like, come on, she's been very obvious.
Like, this is all just stuff spelled out by the narrative and, in some cases, to the characters. What about any of this is confusing?