Maria's nightmares and visions of her birth aren't visions at all, but her memories. She was traumatized at birth so badly that, even though she has perfect memory recall, she doesn't realize it because she suppressed it.
When she found the sonogram, that was when Airam's personality was born. As they are one-and-same, Airam has full recall of every event from birth and is able to quickly piece everything together. Her claims about how Maria and Lily first met weren't lies at all.
It may be that her mother hid that sonogram in the mirror in the hope that one day Maria would find it or simply so that Maria's twin isn't completely forgotten.
Or knew somehow that Airam existed and waited for the right moment for her to fight back against her father—the husband who not only abused her, but killed her sister. Amy was already effectively broken to the point that the only way she could fight back was through her daughter's twin by playing The Long Game.
