I've noticed a series of hits on my Armstrong genealogy page that, from the search engine terms used, appear to be from people researching George Armstrong Custer's lineage.  Several years ago I did some research to find out if Custer was in my own Armstrong lineage (Joseph and Jennet Stewart Armstrong).  In the course of that search I found out that Custer's maternal great-grandmother was a "Sarah Armstrong".  From the dates and locations this might've been the Sarah, daughter of Thomas and Mary Armstrong, mentioned in The Last Leaf.
   Further research excluded her as Custer's ancestor-- she married "Hugh McLelland", which is not a fit in Custer's lineage.  I seem to recall finding that Custer's ancestor Sarah was descended from Joseph Armstrong's cousin John.  I could be mistaken, it's been some time since I did that research.  Unfortunately I can't cite my sources but since there is extensive Custer genealogy info on the internet I'm sure you'll find what you're looking for.