Jaime e Cersei têm tido um relacionamento físico muito antes de irem para King's Landing, como é revelado no terceiro capítulo de Jaime de Uma tempestade de espadas :Did she not find ANYONE else in the entire King's Landing to satisfy her desires?
He could never bear to be long apart from his twin. Even as children, they would creep into each other’s beds and sleep with their arms entwined. Even in the womb. Long before his sister’s flowering or the advent of his own manhood, they had seen mares and stallions in the fields and dogs and bitches in the kennels and played at doing the same. Once their mother’s maid had caught them at it... he did not recall just what they had been doing, but whatever it was had horrified Lady Joanna. She’d sent the maid away, moved Jaime’s bedchamber to the other side of Casterly Rock, set a guard outside Cersei’s, and told them that they must never do that again or she would have no choice but to tell their lord father. They need not have feared, though. It was not long after that she died birthing Tyrion. Jaime barely remembered what his mother had looked like.
Um evento semelhante é mencionado no programa, no entanto, isso não deixa claro quantos anos eles tinham na época. Cersei está muito disposta a dormir com outras pessoas, se ela não pode estar com Jaime, como ouvimos dela ter relações sexuais com Lancel Lannister enquanto Jaime é capturado pelos Starks.
Então, responda à sua segunda pergunta:
When did a lantern go off in her head that got her thinking that doing Jaime was a good idea ?
Algum tempo antes de ambos atingirem a puberdade (10-12) pelo menos.
O livro mais recente, Uma Dança com Dragões , também tem Cersei admitir
That she was also is sleeping with Osmund Kettleblack and his brothers: Osfryd and Osney. Tyrion told Jaime she had been sleeping with Osmund at the end of A Storm of Swords, but as I recall this is just speculation.
No entanto, esses personagens não foram vistos no programa, por isso provavelmente serão cortados.