Now that you mention it🤔 What are the odds that she ate her?
Between chapters 47 & 48, Shiori all but confessed that she loves Hinako -- at least, to the extent that she can. (this is how I read it, based on what I could decipher from the quality of TL work, anyway.)
She wants Hinako to treasure her like she treasures her family, and Shiori admits she no longer wants (if she ever did) to eat her, even though she will uphold her end of the promise to reunite Hinako with her loved ones - but, she asks that Hinako realize that in doing so, how much she'll hurt Shiori (and Miko, that Shiori would rather die than admit that out loud).
In ch. 48, Shiori spies the photo in Hinako's room, playing ignorant in front of Miko that she doesn't understand loving someone so much when you only spent a few years together. Miko comments that Shiori likely does know, because "it's not about how long you spend together that actually matters" - the implication that it's the weight of the moments spent and the subjective meaning within them.
Shiori reflects on her meeting Hinako, admits that she does know, "to the point that it's painful", and then unintentionally transforms when reaching for the photo once she's alone, before reverting and resolving to be the one Hinako treasures most.
And in Ch. 43, that student was getting friendly with Hinako while prepping the ribbons for the cookies for their class's part of the festival, commenting on how happy she was to see Hinako participating and chatting with her about stuff. And the whole time, Shiori was watching from the next table over. Not a lot of dialogue, but the sequence starts on p.10 and goes to about p.17 or so.
So my bet is Shiori is now starting to feel jealous that Hinako
is opening up bit by bit, but not to her - and she's becoming possessive as a result. And I do kind of worry that, in offering to go and help her in Hinako's place in this chapter, Shiori did ....something to "remove the pest", as she might see it.
Whether that was eating her, or doing something else entirely (either as a "human" or as a "mermaid", we can't say yet), Shiori absolutely did something to her to get her to stay away from Hinako.
I do think she likely didn't kill the student; Miko would know something was up before too long, and it's a small town - a student disappearing during the festival would cause a panic, and I don't think Shiori's so dumb as to make trouble like that.
So she might have had a "talking to" to warn off the student to stay away from Hinako or something out of jealousy.