First, I would hope that your parents have a real estate attorney who can best advise them on this issue.
Second, I would think that, like most other liens attached to real estate, this one could be paid at closing. Or are you saying that the lien needs to be Paid in order to obtain an inheritance tax waiver? If so I would think that your attorney and/or title co would have notified you of this sooner. What is your attorney saying?
If you don't have one I strongly advise that you get one asap. Good luck.