Hi Patti and MSC,
It's not really remotely spoilerish to answer these questions, so, in brief:
Sarah in the NoDS story was Sarah Castle, the daughter of the Collinwood housekeeper in 1810 (a character played by our beloved Clarice Blackburn). Sarah does not exist as a ghost in the 1971 sequences, but she is linked to the "present day" in a way that's explained at a pivotal moment in the story.
Selby has a dual role in NoDS: in the present day he is Quentin Collins, an avant-garde Manhattan painter who inherits the Collinwood estate after the death of Mrs. Stoddard. And in 1810, he is Charles Collins, who was romantically involved with a lovely woman named Angelique (who may or may not have been a Witch).
NoDS is often shown on one of the cable channels at this time of year, so perhaps you can catch it at some point.
G.