While it may have been stated that the current Cape Maidens are all daughters of former previous Maidens, this does not always have to be the case. Keep in mind, Nagi's mom left the island when she married and had her son, and was not stated to have had any other children. That would mean that someone had to replace Nagi's mom as Maiden when she moved to Tokyo, and it is not likely to be a close relative, as that would have been mentioned - Nagi's dad would have told him about any close relatives he has on the island.
Then again, there is so much unexplained about the Dragon God and the Cape Maidens. For all we know, while the current Maidens may have been selected from previous Maidens' families, they may have had to select girls from other families due to deaths or inability to bear children.
As I've stated before, my one pet peeve about this series that when Nagi was informed that he was the Dragon God, and that the Cape Maidens were to serve him, he should have gone up to Karin's grandma (who is a priestess of that temple) and ask her to tell him *exactly* what she meant by that, and what is to be expected from all of them. Especially after he found out that he was supposed to give at least one of his Maidens his *favor*, with all the connotations that brings to mind. :mellow: Instead, Nagi is coasting along on whatever pieces of info the Maidens, Oba-san, or Rinnie-chan give him. <_<