In Game of Thrones, did Cersei Lannister not find anyone else in the entire King's Landing to satisfy her desires? At least someone other than her own brother to be the father of all her children? When did a lantern go off in her head that got her thinking that doing Jaime was a good idea? Yes, even though she tells Ned that since they are twin siblings, they are somehow predestined for each other, but still....Why Jaime and Lancel Lannister later, just because he resembles Jaime a bit.
3She just has been around with him for much longer than anyone else she ever met. It's not like she hadn't seen him for 20 years and thought "hey, let me search my brother and have sex with him". It just developed out of an already close personal relationship and that's what makes them predestined for each other apart from their mere physical relationship as twins. And I guess it might as well be due to her (and Jaime) having quite a narcistic character, for which it might make sense to search sex among the own kin, especially the twin.– Napoleon Wilson ♦Oct 27, 2014 at 12:52
Not based on anything in canon, but the entire point that she could have anyone in the kingdom rules kind of rules them out. People tend to want the things they can't or shouldn't have.– corsiKaOct 27, 2014 at 16:59
1@SandySands Yes because we all get to choose who we fall in love with.– Lightness Races in OrbitOct 27, 2014 at 17:51
Well, if you want the technical answer, Jaime and Cersei first pretended to have sex when they were as young as six years old, it might even have been younger. As for the first time they actually did it, it would have been perhaps even the day after Cersei's first period ended (probably twelve or thirteen).– user19511Mar 9, 2015 at 6:22
1Their incest was further 'justified' by comparing to the Targaryens who kept the bloodline pure by wedding brother and sister.– PCARRApr 25, 2015 at 20:45
Did she not find ANYONE else in the entire King's Landing to satisfy her desires?
Jaime and Cersei have been having a physical relationship long before either of them went to King's Landing, as is revealed in Jaime's third chapter of A Storm of Swords:
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.
A similar event is mentioned in the show, however that does not make it clear how old they were at the time. Cersei is very much willing to sleep with other people if she cannot be with Jaime, as we hear of her having sex with Lancel Lannister while Jaime is captured by the Starks.
So to answer your second question:
When did a lantern go off in her head that got her thinking that doing Jaime was a good idea ?
Some time before they both hit puberty (10-12) at least.
The most recent book, A Dance with Dragons, also has Cersei admit
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.
However these characters have not been seen in the show so it will likely be cut out.
2Wow ! That was a good insight. Is that why the prefer the doggy style ? Oct 27, 2014 at 12:21
11@SandySands I actually thought that was a rather poorly-done scene in the show (it’s weird how many of the sex scenes contradict the book). The book emphasizes how their relationship was in many ways egotistical: they look so much like one another that sex with each other was in some ways like sex with themselves. As such, a lot of the point of the sex would have been to look at one another. That position doesn’t allow either of them to really see the other’s face, defeating that purpose. Of course, they also did not quite manage to find an actor and actress who looked nearly identical.– KRyanOct 27, 2014 at 12:47
2@KRyan Wow, that's what I would have thought, too. Didn't know that the books also explain it by their narcistic characters, haven't read those. You might as well put that up as an answer on its own.– Napoleon Wilson ♦Oct 27, 2014 at 12:55
3@SandySands: What do you mean, them being "practically one single person before they were twins"? They aren't identical but fraternal twins (obviously, being of different sex), so they never were "one single person".– DevSolarOct 27, 2014 at 13:02
3@SandySands Oh my! No, that's not what I meant, sorry. I mean that they had to conceal the pregnancy from the audience for the entirety of season one. This is why she is clothed and in that position (it hides the pregnancy). It was not due to safety concerns from un-simulated sexual intercourse. Apr 22, 2015 at 4:58