I have not been able to determine who Sarah's parents are. In the 1790 census, there are two Fisher families listed for Dartmouth: James and William.
There is a birth record in Norton, MA, for a Sarah Fisher, born 20 Feb 1766 to Abijah and Mary, but she may or may not be the one who married Alfred/Alford Rounds. The marriage record of 1784 says she is of Dartmouth.