I've always thought Sansa was a traitor to the House Stark. My memory is fading as the last time I read the book was when A Dance of Dragons was not available yet. Is there enough evidence of her falling to the Lannisters?
At the beginning of the story, Sansa is extremely naive - she fully believes in noble kings and princes who are wise and good, and handsome white knights in shining armor. Getting to live in King's Landing and marrying Joffrey is a dream come true to her. And of course, the whole bigoted society of Westeros reinforces all that. When Joffrey acts dishonest, cruel and vindictive in the incident with Arya and Mycah, Sansa is faced with cognitive dissonance - what she has seen conflicts with deeply held beliefs. She reacts as many do and twists the facts to match her beliefs, shifting the blame on Arya.
Part of that belief system is that Joffrey, as her king and future husband, deserves her full loyalty. Of course her family deserves it as well, but conflicts of loyalty with lethal consequences are not part of her fairy tale education, so when her father tells her they'll go back to Winterfell and thus threatens her fairy tale, she decides (somewhat self-servingly) to betray her father's actions, without realizing what it will lead to.
Technically, that makes her a traitor. But from a purely legal standpoint it is probably actually true that loyalty to the king trumps loyalty to the family, even if the king is a sadistic, spoiled bastard. And from a moral standpoint, she couldn't possibly know just how bad the consequences of her actions would turn out, so she cannot be given full responsibility for them.
The next tragic thing she does is incriminate her father publically, apparently sealing his fate. But this happens under pressure from Cersei, who plays the "loyalty to the king" card unrelentingly, while at the same time promising mercy. Sansa has realized at this point that there's something wrong with this fairy tale, but she has little chance against Cersei, and realistically, the Lannisters would have killed Ned Stark anyway - Sansa just allows them to do it in a more legitimate-seeming way.
Joffrey's decision to have her father executed on a whim after she pleaded for mercy pretty much kills her belief in fairy tales and forces her to grow up very quickly (sidenote: she also soon afterwards has her first period, late for her age - probably symbolic).
Afterwards, she keeps professing her loyalty and love for Joffrey, and calling her father a traitor. But by this time, it's purely an act, necessary to survive. She knows it, Cersei knows it, but is perfectly willing to accept fake loyalty to keep up appearances. Joffrey probably doesn't know - he's not just cruel and vindictive, but also stupid.
Sansa is undeniably the character who develops most over the course of the series.
Has Sansa Stark contributed to the near destruction of her House?
Yes. Her most damning actions were unwittingly revealing her father's plans after Robert's death, and her publicly denouncing her father as a traitor to King Joffrey.
So she's a traitor to House Stark?
Not necessarily. Her first action was the action of a conflicted teenager who was not given all the facts. All she knew was that her father (for some undisclosed reason) was about to destroy her life and dreams. The second act she was manipulated into doing, believing that it was the only way to save her father from death.
Is that all that's against Sansa?
Narrative wise, the fact that she is the only (surviving) Stark with no Direwolf, has made many readers categorize her as the least Starky of the Stark children. Paired with her POVs in A Feast for Crows where she ...
... stops calling herself Sansa Stark and begins taking the persona of Alayne Stone ...
... has only served to further establish this break from the Starks.
There definitely is not enough information to surmise that Sansa has turned against House Stark. Be it to the Lannisters or otherwise.
Here is what George R.R. Martin has to say about the whole beheading of Ned Stark incident and Sansa's involvement:
Your question re Sansa...
The way I see it, it is not a case of all or nothing. No single person is to blame for Ned's downfall. Sansa played a role, certainly, but it would be unfair to put all the blame on her. But it would also be unfair to exonerate her. She was not privy to all of Ned's plans regarding Stannis, the gold cloaks, etc... but she knew more than just that her father planned to spirit her and Arya away from King's Landing. She knew when they were to leave, on what ship, how many men would be in their escort, who would have the command, where Arya was that morning, etc... all of which was useful to Cersei in planning and timing her move.
Ned's talk with Littlefinger was certainly a turning point, though I am not sure I would call it =the= turning point. There were other crucial decisions that could easily have changed all had they gone differently. You mention Ned's refusal of Renly, which was equally critical. And there is Varys to consider, as well as the minor but crucial player everyone forgets -- Janos Slynt, who might have chosen just to do his duty instead of selling the gold cloaks to the highest bidder.
So... all in all, I suppose my answer would be that there is no single villain in the piece who caused it all, but rather a good half dozen players whose actions were all in part responsible for what happened.
Hope that helps.
During her time in The Eyrie Sansa builds a snow sculpture of Winterfell signifying her deep longing for her place.
Of course she wasn't a traitor to the Starks! Once her father had been executed she was all alone in Kings Landing and just a pawn in the political power struggles. Her later behaviour with respect to her aunt was perhaps less honourable - although Lysa was never a Stark of course.
In my opinion, Sansa has merely been the pawn in a political game she does not have the wits or stomach to bear. Without a doubt she contributed to the demise of her family. In fact, I would say it is this reason I despised her so much in CoK and SoS.
But I doubt she ever sought to betray her family. Her decisions at the end of GoT were mostly based off misinformation fed to her by the Lannisters, or her clinging to her girlish fantasies of knights, kings, and highborn ladies. But I think it's clear to everybody that after the first novel, she has very little love for the Lannisters, and especially Joffrey.
Can a 12-year-old girl be a traitor? She certainly wasn't too bright in "A Game of Thrones", and she was easily manipulated by Sersei to tell her what she knew about her father's plans.
In the first book she is a typical love-stricken teen with NO clue about the "real" world other that her fantasy fairytale books about noble knights and kings and other such rubbish. So her view of the world is colored as such.
She learns the really HARD and public* way how the world is and helps in her small ways.
Then she grows and figures out what is needed to protect herself. By going along with Littlefingers she makes sure to live on and maybe get back to her home by the end of it all. So no she is not a traitor to house Stark. She is just a hot headed teen that did the same any other petulant/pouty teen has ever done since the beginning of time.
- Beating, humiliation in public with her clothes torn off etc. Then there is also the matter of her refusing to bow down at the wedding for the cloak ceremony. Feisty and could have gotten her in trouble
Sansa will turn against her family. I'm pretty sure about it. Since the beginning of Season 8, we always caught Sansa talking/planning something with Lord Yohn Royce of the Knight of Vale.
The first was in the Winterfell corridor when they are interrupted by Tyrion, when Sansa told Tyrion, "I thought you're the smartest man".
And the other scene where they are interrupted by no less than Dany, "how about the North" scene. We don't know what they are planning but (correct me if I'm wrong) during the Battle for Winterfell with the Night King, the entire Knight of Vale/House Arryn went Missing In Action.