Go to the Miami County index: http://www.pcdl.lib.oh.us/miami/searchw.cfm
Type in: wiltheiss
You will get 21 hits. These may be her parents, brothers, uncles and etc.
Recheck if William's last name was REALLY McCandlish!
In any case you have a 50/50 chance they came from either FRANKLIN or CUMBERLAND county Pennsylvania.
IF he did, get back with me, I have over SIXTY years research in these, and ONLY these, counties.

Richard Ferman
PS My g-g-g grandmother was Rebecca McCandless from Cumberland County, PA.