One of the episode summaries (I don't remember which one) says "Winterfell prepares for a wedding"
Now, Ramsay hasn't hurt Myranda (yet), so he might very well not hurt Sansa. The show can easily write their relationship as a different relationship than Ramsay-Jeyne and make sense: A real Stark validates Ramsay (which is one of the two things he wants) and strokes his ego, while a fake Stark aims squarely at his issues with bastardy. He was born with nothing. Suddenly, he's not just a Bolton, he's marrying into a beautiful Stark of Winterfell. Is very different than being awarded a steward's daughter pretending to be a real Stark - that can feel like an insult (a fake Stark to a fake Bolton).
As for the risk of bigamy, if the show finds an excuse, why would Roose be bothered with his bastard's offpring risking illegitimacy? He's busy making real Boltons with Fat Walda*. If anything, it's best for him if the legitimacy of any son of Ramsay can be called into question.
And finally, let's assume Sansa is able to turn Ramsay against his father (rather easy) and, one blowjob at a time, turn him into her attack dog. Would she choose, given the opportunity, an incoming Stannis over a manipulated Ramsay? Ramsay is a man of simple tastes after all. She will know what he wants (if anything, Theon will tell her, or it will be obvious). Just like Littlefinger, he can be the devil she knows, while Stannis remains a stranger. And it were Roose and Theon the ones who betrayed Robb. Ramsay burnt Winterfell, aye, but regarding betraying her family, he was just there to pick the pieces.
From actors interviews, Ramsay seems to survive the season and Sansa ends in an ambiguous position. I'm not exactly sure Sansa and Stannis will team up, and I seriously doubt Sansa being rescued in one way or another is a satisfying ending.
*Fat Walda might get pregnant, be killed and Ramsay (or someone else) could say it's a mercy - had the child been born, he might have grown to be a Frey. How would Roose take that?