I think you may be remembering Where Late the Sweet Birds Sang by Kate Wilhelm.
Similarities: Post-apocalyptic, sterility, rich family, cloning, enclave.
Dissimilarity: The enclave is in Virginia, not Pennsylvania.
It won the Hugo Award for best novel in 1977, so that is close to your estimate of 1975.
From the dust jacket copy:
It is the story of the Sumners of Virginia, who have ruled over a lush
valley along the Shenandoah for generations. But the Sumner's land,
their money, their influence do not make them immune to the war and
pestilence that destroy the rest of the earth. Through the foresight
of the elder Sumners, they create a scientific research center that
provides them with a means of survival. But as the Sumner men become
sterile, the women barren, young David Sumner, his cousin Celia, and
the other Sumners become victimized by an army of look-alike,
think-alike clones who are able to perpetuate their Sumner bodies -
but not their humanity. What hope there is for a real, human future
must lie with the outcasts of the new society.
Cover shot from ISFB -