The crack erased Amy's parents from existence. Amy herself still existed because when a crack erases something, that something's effect on the universe, in this case Amy, remains (another example is the Byzantium remaining crashed after the Angels that brought it down were erased by that crack). Amelia was living with her aunt Sharon
when we first saw her.
Now as for where Aunt Sharon was, that was never explained. All we know is that she went somewhere that night and left Amelia alone.