So much going on in this chapter, it's even more confusing than usual... So we learn that Mitsuki is not good at studying, or at least worse at it than even Aya. Aya is self-conscious as usual, and then the twins and even her mom confuse Mitsuki for a guy, as well. Haven't they already met her? If not, it's still strange, since Mitsuki does not even present herself as particularly masculine here, apart from wearing a shirt (which isn't all that gender-specific, anyway, I think?). Also, we have a undercurrent of Mitsuki getting lost in thought every time the graduation exam is brought up, and by Aya's reaction, I assume it has something to do with her decision of whether to move the States or not. Lots to untangle...
The twins have met her, but they're little kids who can barely talk, and they don't really understand concepts like "boyfriend", and they've already associated that term with Mitsuki, so that's what they call her. They're literally just greeting her out of familiarity and it's the only way they know to refer to her. Note how Mitsuki pals around with them once they come up with her, they're all familiar, she's just taking it in stride.
Mom doesn't know Mitsuki, so she mistakes her for a boy, partly because of Mitsuki's androgynous style, but also because her kids just ran into the room yelling BOYFRIEND BOYFRIEND! but then the little ones recount that Aya said she was "an Onee-chan".
Why is it here? Putting aside all the genre-tag business which may or not turn out the way you think it will, this story is here because it’s a fun story with cute characters, unusual and evocative art, and sweet interactions among the cast.
Not every story is designed to meet your individual emotional needs. It’s possible to just read a story for what’s actually on the page and not obsess about what other people say about it.
What Blastaar said.
Yeah, I mean, there's plenty of manga on here that are categorized as het, bi, yuri crush, subtext, etc etc. The suggestion that "why is it even here" like, actively ignores that Dynasty is not, nor has it ever been as long as I've been here, just for romantic yuri genre stories.