Nina's mother is a hallucination. How do we know this? She asks about Nina's rash several times, but when Nina is being measured for her costume, there's no rash visible. She's imagining it, and therefore imagining her mother's presence.
More likely, as with Lily, her mother is real but Nina sometimes hallucinates her when she's not there, or when she IS there, hallucinates her doing things that didn't actually happen.
