The king is greedy, but funnily enough not so greedy he would have his own daughters be used as breeding stock. Yet greedy enough to want to keep the holy attribute as near to the royal family as possible by making all three of his brother's daughters the breeding stock. Too bad that greed does create the problem of producing offspring very closely related to each other. It would be wiser to have Luke have multiple wives that aren't related to each other at all. That way it wouldn't take many generations for them to be able to interbreed without any problems, greatly enhancing the light magic capability. But if the first generation children are not only half-brothers and half-sisters but also cousins, extra steps are necessary further down the line. Though it's also possible the roayls simply don't care.