[@Emma]That's not how it works in history. Her mother would have to be dead in order for her to rule. If Madeleine had a brother, the brother would be ruling regardless if he was younger than her because that was how the world worked back then. The son was always the heir unless there are no sons or any male (related by blood, ie. an uncle or in GOT's case Hand of the King) who could take the throne.

To make it simple, remember The Lion King? Scar killed Mufasa and then tried to kill Simba so that he could be the King because he would be because of his blood. If Simba died, and Scar died, then the Queen would be Simba's mother all on her lonesome. If Nala had married Simba (she would be Queen through him) before he died, and then Simba's mother died, then Nala would be the queen (in the case of being the sole queen/ruler).

Circe Lannister was Queen because of Robert Baratheon, and then he died, but through him she had two sons (supposedly *cough*). Geoffrey and Tomin. Both of them were kings and neither of her daughters were Queen. They remained princesses. But even if her daughters didn't die, Circe would still be Queen and sitting on the Iron Throne (this is also assuming Tywin, The Hand of the King at the time died to and he did, but fact is there are still many many men in the way before a female takes the throne). Not her daughters unless she died or yielded the throne.

Because the world was sexist with their rules.