I believe that she is probably the Mary E. Lightcap who died in the 1870s and is buried in the Levi Lightcap family plot at Pioneer Cemetery in North Judson, Starke County, Indiana.
The Town of North Judson takes care of Pioneer Cemetery. Unfortunately, they don't have any records of who is actually buried there, other than a cemetery index book (Pioneer Cemetery is contained in Vol. III) published by the Genealogical Society. It is based on a walk-through around 2004.
It lists a Mary E. Lightcap having died in 187?. I visited the cemetery for the first time in 2008. Several of the stones for Lightcaps or Lightcap relatives are unreadable.