"...Robert Baratheon, black of hair. Sylvie Baratheon, black of hair. Joffrey Baratheon, golden-head...." Eddard Stark closes the book of The Lineages of the Great Houses of Westeros- he is shocked, and alarmed.
He knows that Jon Arryn's death was mysterious, and now he knows why. Jon Arryn was killed for knowing the truth about the children who had Lannister blood.
Only one of them had both Lannister and Baratheon blood running through their veins, and it wasn't Joffrey the future king- it was his elder sister, Sylvie.
Sylvie Baratheon was the true heir to the Iron Throne. A girl of 18 solar cycles. A girl who listens to her heart and who Eddard Stark thought the truth would scare into running away.
The truth is Sylvie knows what Jon Arryn died for, and she would do her best to do the opposite and stay alive. Acting as though she is not a queen by her birthright.