How long had Tyrion known about the incest?

[They see me R'hollin]

If it were new news to him one would think he would act in a more alarmed fashion, whereas if he knew all along then why didn't he threaten Cersei with it?

[Bright Blue Eyes]

Long. Very long. And why should he threaten Cersei with it? Up until Clash their relationship is cold, but not openly hostile. He loves Jaime, he likes Tommen and Myrcella, he is a Lannister. All doomed if he spills the beans.

[Fat Mac]

Probably as soon as he could understand what was going on he knew. So I'd say likely as early as when he was a teenager.

[redviper9]

If it were new news to him one would think he would act in a more alarmed fashion, whereas if he knew all along then why didn't he threaten Cersei with it?

He's probably known for most of his remembered life. And there's a simple reason why Tyrion never threatened Cersei with the information: Jaime. Tyrion loves Jaime the most of all his family members and is willing -- up until the Tysha revelation -- to forgive Jaime for anything, including incest and throwing seven-year-olds out of tower windows.

Furthermore, Tyrion is a lot like Tywin in that he always puts the interests of House Lannister above everything else. Tyrion knows that the Lannisters' control of the Iron Throne is predicated on the belief that Joffrey, Myrcella, and Tommen are Robert's true-born children. Tyrion threatening Cersei with it would undermine the Lannister cause, which is something that Tyrion isn't interested in doing until Gregor smashed the Red Viper's skull in.

[TheGreyKraken]

Tyrion is a man that tends to find out secrets easily and a secret between his sister and brother would be discovered eventually. He knew about it for a good amount of time, which suggests that he could have learned it while at KL or possibly even farther in the past.

[StarkDragon]

Longer than Kevin.

[Lion of Judah]

Since boy hood I would assume, but certainly once Joffrey was born he knew for sure.

[Naathi Prince]

Tyrion has always known. Cersei and Jaime have been at it since they were teenagers (young teenagers, at that) and Tyrion is much younger than them.

[Black Wolf Smith]

I guess I am the only one that thinks he did not know. They are 8 years older, and Jaime was a squire and then knighted by 15, and then straight into the KG. Tyrion was at most 7. Cersei was gone not too long after, and they never really talked. I wouldn't think he knew until he heard it, then it clicked.

[Lummel]

Tyrion implies that he is aware of it in Tyrion I AGOT when at breakfast with his siblings. As to when he first came aware of it who knows.

Cersei in one of her POVs writes how she and Jaime were inseparable as children and since being close to her was Cersei's inducement to Jaime to join the kingsguard they presumably were in a relationship since Tyrion was circa six /seven. He would have had plenty of time to observe, talk with his siblings, see the birth of his nephews and niece and draw his own conclusions.

ETA

If it were new news to him one would think he would act in a more alarmed fashion, whereas if he knew all along then why didn't he threaten Cersei with it?

Tyrion couldn't threaten Cersei with it because he was a part of the lannister family and more importantly because of his relationship with his father, were up until the end of ASOS he is still looking to him for love and validation. As an example of just how extreme Tyrion's state of mind is on this issue when Tywin denies him the succession to casterly rock, Tyrion accepts it (with bad grace) as a fact rather than seeing this as the idiocy which it is.

So from Tyrion's POV prior to the end of ASOS making public the Twincest would not just bring down Cersei but also the whole of house Lannister. It's effectively suicide from Tyrion's POV and all the more dangerous to him because he's a dwarf. All of his power and status is caught up in being a Lannister, unlike his brother Jaime he fears that his skills won't be enough to be make it in the world on his own without the honour of the lannister name intact.

[Runaway Penguin]

tyrion knew but ddin't care much and he loved Jaime - and Tommen and Myrcella. He got into conflict with Joffrey not because of twincest, but because Joffrey was off his rocker.

[Lyanna Stark]

Tyrion implies that he is aware of it in Tyrion I AGOT when at breakfast with his siblings. As to when he first came aware of it who knows.

In light of this, it's really interesting that people don't take more notice of how Tyrion is all nice to the Starks, but suspects his siblings of causing the Bran incident, yet however he does not try and actually tell someone who look into it further.

[Sansa_Stark]

In light of this, it's really interesting that people don't take more notice of how Tyrion is all nice to the Starks, but suspects his siblings of causing the Bran incident, yet however he does not try and actually tell someone who look into it further.

And kill his brother and sister(well he'd like that) and all her children?

He's being two faced of course, but there are reasons why he can't tell.

I never understood why Jaime and Cersei just left Bran alive like that though.

He could have easily just woken up and spilled the beans.

[Lummel]

...I never understood why Jaime and Cersei just left Bran alive like that though.

He could have easily just woken up and spilled the beans.

Well we do see Jaime and Cersei's response to the news that bran might live in Tyrion I AGOT, but really they don't have the opportunity to polish him off in Wintersfell, it's not like King's landing where the Maester is a Lannister groupie and could slip a little something into his medicine.

ETA

In light of this, it's really interesting that people don't take more notice of how Tyrion is all nice to the Starks, but suspects his siblings of causing the Bran incident, yet however he does not try and actually tell someone who look into it further.

What can I say? Clearly not everybody feels in their bones that Tyrion's moral compass points firmly and unflinchingly towards casterly Rock :laugh:

;)

[Woman of War]

In light of this, it's really interesting that people don't take more notice of how Tyrion is all nice to the Starks, but suspects his siblings of causing the Bran incident, yet however he does not try and actually tell someone who look into it further.

But whom should he trust with that knowledge and yet be sure that this helpful investigator won't endanger the life of Tyrion's family? The murder of of Tommen and Myrcella would have been collateral damage. Tyrion could try to investigate himself but for sure the reason that he is even interested must never ever be known. And, frankly, caring about the wellbeing of niece and nephew should be paramount. Tyrion is justified in putting their life above whatever lawful justice. Immoral? The three kids would have been dead meat otherwise, Robert was still alive.

[Lyanna Stark]

And kill his brother and sister(well he'd like that) and all her children?

He's being two faced of course, but there are reasons why he can't tell.

I never understood why Jaime and Cersei just left Bran alive like that though.

He could have easily just woken up and spilled the beans.

Oh, I am not saying I don't see his motivations, but it does put him as squarely aligned with the Lannister, and not a Ned Stark type good guy which many seem to want to cast him as. And when people wax lyrically of how great Tyrion was to Bran when he made him the saddle, well, wouldn't it have been greater to actually bring the truth? Tyrion claims to be a friend of Jon Snow, but isn't it two faced to be his friend, while knowing his siblings tried to murder said friend's younger brother?

[Winter's Knight]

Oh, I am not saying I don't see his motivations, but it does put him as squarely aligned with the Lannister, and not a Ned Stark type good guy which many seem to want to cast him as. And when people wax lyrically of how great Tyrion was to Bran when he made him the saddle, well, wouldn't it have been greater to actually bring the truth? Tyrion claims to be a friend of Jon Snow, but isn't it two faced to be his friend, while knowing his siblings tried to murder said friend's younger brother?

Lyanna, people still consider him to be sympathetic to the Starks after he married Sansa-it's a bit much to ask that they pick on this much subtler point.

[Kittykatknits]

Oh, I am not saying I don't see his motivations, but it does put him as squarely aligned with the Lannister, and not a Ned Stark type good guy which many seem to want to cast him as. And when people wax lyrically of how great Tyrion was to Bran when he made him the saddle, well, wouldn't it have been greater to actually bring the truth? Tyrion claims to be a friend of Jon Snow, but isn't it two faced to be his friend, while knowing his siblings tried to murder said friend's younger brother?

Lyanna, people still consider him to be sympathetic to the Starks after he married Sansa-it's a bit much to ask that they pick on this much subtler point.

I gotta agree with WK on this one. A few inner thoughts of his seems to indicate some great sympathy for the Stark family. Yet, his actions and justification for those actions show us he is very clearly aligned with the Lannister cause. He's not a Ned or a Davos, that's for sure.

[WardenOfTheNorth]

If it were new news to him one would think he would act in a more alarmed fashion, whereas if he knew all along then why didn't he threaten Cersei with it?

He's probably known for a long time. But he doesn't want to hurt Jamie, so he doesnt do anything with the knowledge.

[Lady Sansa Stark]

Why would Tyrion spill the beans here? It would mean his death, that of his sister, his beloved brother, their children with an emphasis on Tommen and Myrcella and his family would be in disgrace forever. They would lose everything.

Ned Stark would've done the same. Davos Seaworth as well. No one is going to risk their beloved one's lives for that. (see how I conveniently forget Rodrik Cassel here?)