Naming of characters by characters

If she's the POV character (assuming 3rd person limited), then use whatever names she would use. Putting "grandma" in quotes before she knows this isn't really her grandma implies that the narrator knows something the POV character doesn't, so that's a POV violation. It's too much of a giveaway to the reader that hey, there's something wrong here, but she doesn't know it yet.

When she realizes these people aren't who she thinks they are, how will she change? How will she refer to them? I could see after the revelation having the narrative refer to "grandma" because at that point she knows the name is wrong. Maybe she switches to mentally referring to them by their given names?

i suppose it depends on when you the the cat out of the bag about the fact they are not her real family...I would say grandma without quotation marks

I'd make Mom, Grandma and Grandpa proper nouns.

Grandma changed the sheets yet again, mumbling complaints all the while, as Grandpa watched from the doorway, a disapproving scowl on his pinched face.

-Derek

TheIT is on the mark here; since the girl is the POV character, the grandparents should simply be called the way she would call them before she knows who they truly are. No quotes. (Using quotes would imply that the girl has knowledge she doesn't have yet, that her grandparents are fakes.)

As for what she calls them afterward, that too is a POV issue. You'll need to think over what your character would call them. Put yourself in her shoes. Would she call them "the impostors"? "Fake grandparents"? Would she give them nasty names like "Those lying bastards"? Or would she still call them "Grandma" and "Grandpa" just because she herself has no clue what to call them now and/or she's so used to calling them that? (E. g., "She turned to look at her grandmother--fake grandmother, she had to remind herself...")

It's up to her to name them; see what she thinks.

I agree with Derek and tehuti88....

Maybe in the beginning you could call them Grandpa/Grandma then after the secret is out call them grandma/grandpa. I think this would be more subtle but still easy for the readers to follow. Good luck.