What I understand is that the quints are called by their 1st names, in situations where they would normally be called by their last names, for obvious reasons (that they're quints. Extend in general to half/step siblings of the same age an last name).
In the 1st episode, we see that 9-year old Raiha calls 16 or 17 year old Itsuki as Itsuki-san (or Itsuki nee-chan in the manga) instead of Nakano-san (or Nakano nee-chan). But it seems Raiha doesn't know about the other 4 quints until S01E04. (See below.) So what's going on? Guesses:
Itsuki's just being friendly to Raiha.
Or Itsuki in fact does tell Raiha about the 4 quints, but Raiha forgets about the other quints when seeing what simply looked like 4 doppelgängers.
Or maybe out of obvious lifelong habit Itsuki just tells people to use 1st name.
Edit: I'm actually not so familiar with all these honorifics and stuff. What I'm asking really is like
Do we really expect that Raiha to call Itsuki as Nakano-san but then for some reason Raiha doesn't?
If no, then why? If yes, then why the change?