When Jon and Sansa first reunited in Season 6 of Game of Thrones, it felt like a major victory. None of the Stark kids had seen each other in years, and in the interim they had been subjected to all manner of horrors. Sansa in particular had endured intense abuse at the hands of Joffrey and Ramsay, so when she arrived at Castle Black, it was a relief. Sansa and Jon were finally with family again. But the joy of their reunion was short-lived and it looks like things will only get more fraught from here on out. A Jon Snow and Sansa power struggle is happening on Game of Thrones Season 7, and it's been building for a while.
Jon and Sansa never particularly got along when they were growing up, so it's not really a surprise that they would continue to butt heads in adulthood. The seeds for that future conflict were sown in Season 6 shortly after they reunited, when they immediately had to galvanize for the battle against Ramsay. They both made missteps there: Jon didn't listen to Sansa enough and she kept the mid-battle arrival of the Knights of the Vale from him. Though they both expressed more than once that they had to trust each other, it was clearly going to be a rough journey. And then Jon was declared King in the North.
That could end up being a major problem for the recently reunited sibling-cousins. As far as anyone knows, Sansa is the last remaining heir to Winterfell, yet she has seemingly had her power usurped once again by Jon suddenly being made a king. Even in her own home, the one place where she should be assured the control long-denied her, she is set aside for someone else. While Jon's intentions are absolutely not malicious, it's still easy to see how this could breed tension — something that Littlefinger is primed to take advantage of.
Kit Harington spoke about the problems between the pair in an interview with Entertainment Weekly. "There is definite tension between them right from the first scene," he said, referring to the upcoming Season 7. "There are the same problems — she questions his decisions and command; he doesn't listen to her. But as far as where that goes or takes them or how dark it can get, we'll see. It gets past sibling squabbling, it gets into two people power struggling."
It would be tragic for two of the remaining Starks to be pulled apart by issues of power, so hopefully Jon and Sansa remember they're family and find a way to work things out. No good ever came from listening to Littlefinger.