I'm not sure Sansa really made sure of anything .... she just sat there.
She did smile, and that's understandable given that everyone is willing to unite behind the Stark banner. I assume her biggest worry wasn't whether or not they would unite behind her but whether or not they would unite behind the Starks at all. And at least for the moment, uniting behind her and Jon effectively is the same thing.
But yeah... they likely are trying to build tension between Sansa and Jon, and I'm sure Peter will do everything in his power to stir that up. She also likely knows that Jon is threat to Peters goals now... down the line that's going to be a major problem. Whether or not Sansa ends up adding to the problem or helping to resolve it I imagine will be her arc for the remainder of the show.