As of the end of Season 6 of Game of Thrones, it seems very likely that Daenerys and her fleet/army will, at some point, come into contact with Jon Snow and the North. It's entirely possible that they'll become allies - they both want the Lannisters removed from power, and they have a mutual friend in Theon Greyjoy.
As Daenerys said, "The best way to make alliances is with marriage." I know there's been a lot of speculation about a possible marriage between
Daenerys and Jon.
But what about the fact that Daenerys's Hand of the Queen has already been married to Jon's sister? And the relationship between Tyrion and Sansa, while not exactly loving, was better than how Tyrion and Shae's time ended, and certainly went better than Sansa and Ramsay's marriage. So the two of them might be amenable to the idea of marrying for the sake of their respective factions.
But a lot has happened since then - Sansa was married to Ramsay, and Tyrion was convicted of murdering his nephew and fled the kingdom after killing his father. And the lords of the north might not be so keen to kneel to a Lannister. If the marriage is no longer seen as legitimate by the populace then it would defeat the purpose of sealing the alliance.
So my question is: Assuming Sansa and Tyrion want to resume their marriage, is there any law, custom, or precedent in Westeros that could prevent them from doing so?