Okay, now I get it. Well, no one has really questioned the rule, that the first born will inherit the thrown. And Naruhito was older than Akishino. Likewise, the first born's firstborn is also the heir, even if there are cousins born before him. Therefor, Crown prince Frederik's son will once become the king of Denmark, even though Frederik's younger brother Joachim had children before him. No one thought Mako and Kako had more right to the thrown than Aiko, because Aiko was the crown prince's daughter, and Mako and Kako weren't.