Now that we have related the manner of marrying amongst the Indians generally, it will be well that we should describe the customs relating to the marriage of the prince who was heir to the kingdom. It must be known that the Kings Yncas, from the first, established it as a very stringent law and custom that the heir to the kingdom should marry his eldest sister, legitimate both on the side of the father and the mother, and she was his legitimate wife, and was called Ccoya, which is the same as queen or empress. The firstborn of this brother and sister was the legitimate heir to the kingdom.

They kept this law and custom from the time of the first Ynca Manco Ccapac and his wife Mama Ocllo Huaco, who came saying that they were brother and sister, children of the Sun and Moon, and so all the Indians believed. They also had another ancient precedent to justify this first one, which was that, as has already been said, they believed, in the time of their heathenry, that the Moon was wife and sister of the Sun, from whom the Ynca was descended. Hence it was that, in order to imitate the Sun in all things, the first Yncas and their descendants established the law that the first born of the Ynca, following both these precedents, married his sister both on the father's and mother's side. In case of failure of such sister, they married the most nearly related cousin, or niece, or aunt in the royal family, and, on failure of male heirs, she might have inherited the kingdom, as in the laws of Spain.