Because she was born into a normal, present-day setting, which has no way of accessing this other dimension or awareness that it exists, these are considered to be hallucinations rather than any sort of legitimate perceptions. She's also gifted, which brings with it some social problems (primarily that she doesn't much care for her peers, so she doesn't really bother with trying to socialize with them), and has an introverted personality. However, the "hallucinations" are the only symptom of serious mental dysfunction that she suffers from, and they don't really change in "severity", go away or appear less frequently with medication, or influence her behavior - the people in the room don't even realise her presence, let alone tell her to do things; the only way it affects her is that she sometimes reacts to them in a similar fashion to the way people react to movies and TV, and they've been known to distract her from other, more important things. Finally, she refuses to accept the proposition that she's mentally ill, although she doesn't share her alternative explanation with anyone for fear of ending up being considered delusional on top of everything else.
I'm not sure what she'd be diagnosed with; although paranoid schizophrenia seems pretty close, I don't know if the nature of her perceptions would match up with the standard criteria for delusions and/or hallucinations. Furthermore, although this doesn't seem to me like it'd be severe enough to warrant it, I need to know if she would have much likelihood of having been institutionalized at some point in her life.
I've tried Wikipedia's entries for schizophrenia and hallucinations, but it's not really something that I know how to google (if it can be googled at all), and haven't found a specific enough answer to be sure about it.
This isn't a major plot point, by the way; I just want to know for the sake of giving a bit more detail to her backstory.
ETA: It appears that there are a few plot holes in this that I didn't catch; I was intending for the family to have found out when she was too young to realise that she should keep it to herself, but I didn't account for the fact that most parents would probably have written it off as an overactive imagination. However, since being diagnosed as mentally ill is a fairly important part of her character, I'd like to know what changes I can make in order to keep that plausible.
ETA #2: Alright, looks like I need to rethink this a bit; thanks for your help, everyone.
