This is a story about twins. About a connection between them that is even stronger than mother-child connection. A pair of twin, as long as they have each other doesn't need other people, doesn't need a connection to other since they're a complete single entity, unlike other person who is only an uncomplete half searching for the other half. At least this is what the book wants to convince you. My advice for you... don't hold on to the twin idea too firmly, there's more to the mistery than what she wants you to believe.