OUAT!Princess and the Pauper AU:
Snow White gives birth to twin girls but the babies are immediately separated after Rumplestiltskin, who knows that any product of true love is bound to be powerful, steals one of them.
As a result, the twins grow up in two very different worlds. One of the girls is raised by the King and Queen and becomes a traditional, graceful princess. The other one is raised by Rumplestiltskin and becomes the Dark One’s top student.