This is where you'll need professional involved. Did you have a home inspection? And if you did, why didn't he/she see the crack? Assuming you knew about it then, EVEN if it did damage for the last year, the courts may say you lost your opportunity to fix/avoid the problem back at the time of the inspection. (I'm a hard-butt when it comes to spending a few hundred $$$ for a home inspection when you're gonna be spending 100's of 1000's of dollars on a home!)
If you didn't have the home inspected, you'll need a Structural Engineer to assess the situation. Assuming he says the crack HAD to have caused the 'problem' with the window 2 years (or more) ago, now you have to prove if the previous home owners knew about it. Maybe he can find evidence of water in the house where they HAD to have known the cause was due to a crack. He'll have to look for the tell-tale signs of a 'cover up'. DON'T go doing this yourself. Let the Engineer do the work to uncover any evidence. You may destroy evidence if YOU start poking around. IF there was a Seller's Disclosure Statement and they said they didn't know about the crack and YOUR engineer says/proves they had to have known, then you have a case.
Now, assuming (again) they DID know and they lied, how are you gonna collect the $$$? If they're in another state, it gets hard. If they're elderly, good luck again. You might want to do this: Get a Structural Engineer out to see what it would cost to fix the ENTIRE problem, all the way around. If it's less $$$ than retaining him for court, lawyer fees, and so on, then spend the $$$ on the fix. (Case Closed ...) My best to ya and hope this helps.