The last reference I find to Penny Seeger in the New York Times is an announcement of an event where she will perform that was published on July 7, 1991.

http://www.nytimes.com/1991/07/07/nyregion/westchester-guide-934591.html (Scroll down to fifth paragraph from the bottom.)

See if this will load: a PDF of the obit of their father, Charles Seeger, age 88, published February 8, 1979. PDF from the New York Times archive.

SRS