Typically, the baby that comes first out of the womb of the mom is considered to be the elder of the two twins as its the one who comes into the world first. But, as mentioned in the first chapter of the above book, the child that comes out second is considered to be the elder of the two twins in Japanese tradition. The logic being that the child that comes out second waits and allows the younger one to exit first.
I think this is a refreshingly beautiful example of an alternate world view. A world view based NOT on competition but cooperation.