This is a very annoying thing that often happens when one partner dies. Spouses go back to fiancés, boyfriends/girlfriends to romantic interests, etc., so what happened to your couple would also be a direct result of the death. As for them not being able to remarry, you could try breaking them up by using several 'mean' interactions, then working your way back up again. I don't really know why they can't just get married again, but I have heard it is very common...
Good luck!