Well, as Yukari's "weapon", Ran would've been in the (extreme) front row of Kaguya's defeat during the events of Imperishable Night (assuming a Reimu playthrough), so she should know all about Kaguya/Mokou.
In fact, it's a little strange that Nitori wouldn't know, being as information on who Kaguya was should have disseminated through some of the canonic cast herd get-togethers, like Silent Sinner in Blue, even if only through the Tengu like Momiji.
This is much later than the previous comments, but I don't think this is completely following canon. In the beginning, Nitori seemed to speak to Reimu and Marisa like she had never met them, and they didn't appear to know her either.
Kaguya.Does that human truly bear that name?Umm, yes.Moko....Hmmmm.Furthermore, the young child living with you is....Eh? Umm, yes....And lately, she's been visiting Kaguya's house pretty often....Not good.Are you absolutely certain that her name is Mokou?