So, assuming that Sansa will probably have some role in taking Littlefinger down, how do you see Petyr's fall going down in later books?
Well, I think Littlefinger’s going to overstep Sansa’s moral boundaries in a way she cannot ignore. Whether that’s Sweetrobin’s murder, learning what he did to Jeyne Poole, putting together what he did to her father, or some combination of the above, I think she comes to the realisation that she has to do something.
I’ve thought for a long time that Sansa will manage to bring Littlefinger to trial for at least some of his crimes, and yes, I think the show drew on the books here. Not sure what crimes in particular she’ll get him for, but she’s an eyewitness to Lysa’s murder (though she’ll have to explain how Littlefinger pressured her to lie about it afterwards, without losing her own credibility), and she heard Lysa’s confession regarding Jon Arryn’s murder. If the dagger Littlefinger’s carrying around is the same knife that he lied about to Catelyn and Ned, with any luck she can pin the start of the War of Five Kings on him.
No strong theories about exactly how she brings this trial about, but I think she manages the case and counters his efforts to dodge guilt with the very skills Littlefinger’s been teaching her, in order to ensure the trial is fair.
She wondered where this courage had come from, to speak to him so frankly. From Winterfell, she thought. I am stronger within the walls of Winterfell.
I also think that this is only possible after Sansa establishes and re-establishes her support networks in Winterfell.