The WW is the big outside factor, but that benefits Jon in terms of influence, not Sansa.
He has done plenty (showing up in the nick of time, on Sansa's request, with the Vale army to regain her favors), but right now he knows he has to regain her trust by pitting her against her brother.
And judging by this last episode (and her face last season when they started calling Jon King in the North), it might be working.
If she truly thinks she can manipulate him, her downfall will be even more glorious to witness. Specially if said downfall results in her betraying her own family, in spite of saying she was now in control (because obviously playing into his game).
Here's the thing - first of all, I don't think Jon will stay at Winterfell after the next episode. He's gonna get 2 ravens probably - one from Dragonstone demanding he bend the knee just like the one he got from King's Landing, and one from the Citadel telling him
about Dragonstone. That's a massive incentive, literally, so he's gonna wanna go over there asap so Sansa won't have Jon around to manipulate.
The other thing is, we saw only one scene with Bran and that was at the wall, but I don't think he will stop there. It would make sense for him to want to go home to Winterfell, as dolorous Ed will tell him that his family has reclaimed it. And with Bran there, Sansa is reunited with another brother, which will make it harder for Carcetti to whisper to.
Pretty much why I hope they don't go that route. We've less than two full seasons left and the last thing we need is another smarmy, sociopathic asshole to cause trouble. Ramsay was invincible until the Battle of the Bastards.
As evil as he may be, I think Euron's armada, coupled with experience and psychopathic nature will work in his favor for whatever "gift" he's gonna bring back for Cersei.
He did save Jon during the Battle of the Bastards, and he still has the Knights of the Vale which is a pretty damn good card to have in your deck.
I think Sansa should consider marrying Robin. That would give her power of the Vale over Carcetti, yeah?