Twin Theory is something that Grace nicknamed Castiel's theory about how two girls can lookalike and have come from the same person but each girl has a different set of parents. Like Grace and Holleigh.
As Holleigh and Grace were born the image of each other, Castiel stated it maybe natures way of dealing with the birth of a Nephilim as the temporary hosts of Lucifer and Amelie had been trying for their own child before being taken over by the Angels. Grace stated it was ridiculous that nature would try to fix its mistake by making twins; one biologically the child of the Angels and the biologically human. To which Castiel remained certain that it was possible as her existence was rare in itself.