I wouldn't say that Nancy is a regular attendee but she has been to several over the last few years. In addition to those that you mentioned, Marie Wallace almost always attends. Donna Wandrey, Jerry Lacy and James Storm have been to a lot too. Kathleen Cody has been to a couple. Donna McKechnie still works pretty regularly, so she hasn't been to a festival in some time.
Sharon Smyth attends almost all of the East Coast events. She was at the 50th. Denise Nickerson was not. She hasn't been at a DS event in many years.
Festival guests are almost always from the original series. They did invite some of the NBC cast in the early 1990's (Barbara Steele, Adrian Paul, Roy Thinnes, Jim Fyfe, Barbara Blackburn, Lysette Anthony, Michael Cavanaugh, Veronica Lauren, Juliana McCarthy and Joseph Gordon-Levitt) but not in years. Fyfe and Joanne Dorian attended a smaller, non festival event last year.
As far as family goes, there's been a few. Matthew Hall, Sam and Grayson's son, was at the 1989 festival. One of Joan Bennett's daughters (Diana) attended a California festival about 10 years ago to promote a biography on the Bennett family. Jonathan Frid's nephew Don was in Tarrytown this summer.
I think that's everyone.