The construct of the doctrine of DF'ng sees it as a forgone fact that shunning will bring the person "to their senses."
What if, however, the person will never be brought back? Then continued shunning is logically an effort in futility.
Maybe this appeal to logic will help in your parents case. What your father has to overcome though is the very real possibility that he will at the least lose his position within the cong. and quite possibly be DF'd himself if he refuses to shun you.
Loving Christianity at it's best indeed.