For years, fans have speculated Ramsay is a liar, and Benioff and Weiss ironically confirmed as much since they revealed that Martin told them he planned to have Stannis sacrifice his sweet daughter Shireen Baratheon to the Lord of Light. And on the page, Stannis left Shireen, along with his wife and red priestess Melisandre, at the Wall where Jon was murdered.
All of which is to say that rather than Jon awakening and finding a cut and dry reason to avenge his own death and then be encouraged by his long-lost sister Sansa to retake Winterfell, he’ll find a political maelstrom in which a supposed ally will return to the Wall and attempt to sacrifice a little girl—and his own kin—under Jon’s protection. The snowball effect of these different circumstances could be significant since Jon will have command over loyal men of the Night’s Watch, free folk, and even some Baratheon men when this comes to a head… particularly since, according to Ramsay’s letter, Stannis’ retreating army will contain what’s left of Theon Greyjoy (the “Reek” Ramsay demands back) and a woman who claims to be Jon’s sister….
One of the biggest points where Game Of Thrones intentionally diverged from “A Song of Ice and Fire” is when he had Sansa Stark return North to marry Ramsay Bolton. On the page, she has yet to set foot back inside Winterfell despite her longing for her lost home. Rather it’s Sansa’s childhood friend, Jeyne Poole, whom the Lannisters and Boltons are attempting to pass off as Arya Stark (whom they presume is dead), and therefore key to supposed legitimacy in the North.
Yes, on the page the character remains a young girl whom Theon/Reek and the readers knew when she was growing up. Ramsay also still does monstrously unspeakable things to her on their wedding night (as bad as the scene is in the show… it’s worse in the book). However, she is neither Sansa nor Arya, meaning as far as the North is concerned, the only living Stark child is the bastard Jon Snow—at least until the other living four are found.
Further, unlike in the show, the North truly remember what happened at the Red Wedding. While Stannis might’ve been defeated by the Bolton forces at Winterfell in the book, most of the Northern lords who seem to begrudgingly accept Roose Bolton’s authority as Warden of the North are perpetuating a conspiracy to betray the Boltons. Lord Wyman Manderly, the portly patriarch of White Harbor, heavily implies to Reek that this is a “murmurs farce” that will soon end, and the book would seem to suggest Manderly has already killed several Freys and men who participated in the Red Wedding and served them as meat pies to unsuspecting fathers and Boltons during Ramsay’s wedding feast (which Game Of Thrones adapted into a direct Titus Andronicus shoutout with Arya feeding the most guilty Frey sons to Walder Frey in the show).
So to recap: the Boltons think they hold Winterfell while being surrounded by enemies within, Stannis survived his defeat and is headed back to the Wall, presumably with Theon, and Jon Snow will be resurrected in time to see Stannis attempt to sacrifice his daughter to the same red god who beckoned him from beyond the grave.