Sorry if you answered this before, but how and why do you think the incest started between Jaime and Cersei?
Hi! Wow, I wrote way too much for this. I think first a timeline and some text will be useful before we get started.
266 - Jaime and Cersei are born at Casterly Rock
272 - Jaime and Cersei are taken to KL for Aerys’s Anniversary Tourney, though Cersei does not see Rhaegar there. “When she was just a little girl, her father had promised her that she would marry Rhaegar. She could not have been more than six or seven.” (This art by @bidonica is my favorite thing.)
273 - Jaime and Cersei are seven. They are discovered by Joanna’s maid. Joanna dies soon after. After the Martells arrive, Cersei sexually abuses Tyrion by twisting his penis; she physically abuses her baby brother on more than one occasion.
276 - Jaime and Cersei are 10. Tywin hosts a tourney. Maggy the Frog tells her prophecy. Cersei sees Rhaegar for the first time. Aerys refuses the Cersei/Rhaegar betrothal and he refuses Jaime as Rhaegar’s squire.
277 - At eleven, Jaime is sent to squire for Lord Sumner Crakehall.
278 - Tywin brings Cersei to court. She is 12. Cersei wept and Jaime raged.
279 - Jaime wins a squire’s melee, though where this melee took place is unclear. Possibly at the tourney held at Storm’s End?
279 or 280 - Tywin visits Casterly Rock with Cersei. Jaime visits home too. The twins are 13 or 14. Jaime dares Cersei to kiss Tyrion, which she does. Cersei laughs at Tyrion’s tumbling, until Tywin puts a stop to it.
281 - Jaime wins his spurs against the Kingswood Brotherhood. Jaime and Cersei are known to have slept together at Eel Alley. They are 15.