[X] Answer truthfully that we only know the scheduled wedding will bring the foretold doom. Recommend that rather than making a hasty decision now, they instead postpone the wedding. Another Divination at some point in the near future, perhaps six months or a year from now, could be performed to verify the results. If doom is once more the result, they will know the union is cursed.
[X] Agree that it is likely the timing of the wedding, not the joining itself, that would bring the doom you saw. After all, you did try and it is not as if the prince can ask back for payment he can't admit to have given you.