But the Boltons wouldn't want to marry Sansa, she's a suspected regicide!
Do the Boltons know?
Okay.. ravens, but seriously.. First of all, and this is a broader peeve so let's deal with it first, the ravens make absolutely no sense. You may as well go through the entire book series and replace all mention of "raven" with the word "cellphone" because that's what they are. They're a magical way for people to communicate over long distances when they shouldn't be able to.
Carrier pigeons work because some breeds of pigeons have an innate instinct to return to the same nest in order to breed. Precisely how they do it is still not completely known, but regardless they can and its pretty amazing and was very useful at one point. Ravens are likewise pretty amazing animals which can perform many feats of intelligence, but they a) do not have a homing instinct and b) even if they were some magical breed of homing ravens, would not be able to understand where they were supposed to go. Unless a maester has a raven trained to fly to every location in Westeros, they're not going to be able to use them to communicate in the fashion they are shown to.
Secondly, suspending disbelief on the magical corvids, has everyone been told that Sansa is a suspected regicide? To my memory (which may be wrong) it doesn't seem to have come up in any conversation that this is public knowledge, furthermore, even assuming it is public knowledge down in King'd Landing, why would the Boltons (or anyone in the North) be told? What's the message going to be? "hey, you know that girl who is technically the rightful heir to your stupid "king in the north" title and who we'd effectively been keeping hostage.. well, we lost her. I guess we're dumb like that!"
If you're going purely by rational logic, it makes no sense to tell anyone. Tyrion took the fall, and his assumed responsibility at least makes the Lannisters look semi-competent as well as getting rid of a family member noone liked. Even Cersei doesn't actually seem interested in the idea of bringing Sansa to justice so much as just having her tortured and killed, and generally you don't need to tell anyone you're doing that.
Sure, this is speculation here.. but let's consider one thing. The Boltons don't seem to know that Sansa is already married, which means they presumably don't know that Tyrion is alive.
Maybe something in the books contradicts this, but then.. cellphone ravens. Books can be pretty dumb too.
Also in "things in the books that totally don't make sense", Dorne. Just.. Dorne. A fiercely independent country whose own succession system basically ensures it would, in reality, almost certainly cease to be independent within a few generations, because inheritence =/= lineality.