Hmmm ... Grandma was demanding to know what Kaya did to Nana ... but we see that Auntie Nana alive, seemingly with nothing wrong with her for now. This seems to point towards the fact that Kaya is either an extremely dangerous and powerful entity herself, or is possessed by one or she has like a "dark" side or entity part to her. Evidence so far seems to point towards Kaya having both a human side and some entity side to her. Kaya clearly exhibits human emotions, and she also has some fears about being seen as a "monster." Ironically, it seems that while she may be perceived monster in the eyes of kids who don't know why she is acting the way she is ... simultaneously she actually does have some kind of monster part to her.
I don't have a clue what's going on with Nana and the mother though. It seems like this this auntie Nana is a real person, but then again we really never know. Some monsters seem to be able to reveal themselves to regular people, like the parent impersonating entity did. So we can't conclude that Nana is human just because other people have acknowledged her presence. We don't even know if she really is Nana for that matter. What grandma said made it seem like Kaya's mom was supposed to be Nana. And speaking of Kaya's mom, we really don't know about her true identity either. Mob seems to have a theory about her being pregnant with an entity or at least a baby that's possessed by an entity, and Chie-sensei also have seen Kaya's mother first hand. Unless these entities can control corpses in a way where the body doesn't decompose and can somehow show signs of life like having a pulse, breathing, etc., it seems like Kaya's mom is actually alive. But grandma believes that she's dead. A lot of information here seems contradictory.