Mercy is a puzzle. She appears as "Mercy Penderson" on a list of John & Sarah Turner's children in John's US Military Pension committee file (see attachment) 
An 1828 marriage record is found near Marengo, Galen, Wayne County: 
Now, is this John & Sarah's Mercy? Perhaps she has a previous marriage?
↑ Revolutionary War; military pension committee notes: children of John & Sarah Turner
↑ Geneva NY Gazette of November 5, 1828 reported that William Punderson, formerly of Junius, married Mercy Jenny, of Lyons (Wayne County), near Marengo, Galen, Wayne County / found at Genealogical Publishing Co.; Baltimore, Maryland; 10,000 Vital Records of Western New York, 1809-1850