I originally had $15K in student loans, same deal as your fiance, parents were divorced and Dad was supposed to pay it back. He did, 2 weeks after it came due, all I had to pay was the 2 weeks of interest.
But for all of that, I was saving up and prepared to start paying it down myself. Any reason why you can't pay it off, then if they pay you back, great, but if not just keep moving forward and don't stress about it? I'd hate for money to come between you and your future in-laws.