You probably already have this information but I thought I would post it just in case you didn't have it.
I located the parents/family of Foster Casner on the following census records:
1880 PA, Juanita Couty, Milford Township 1900 PA, Juanita County, Spruce Hill Township 1910 PA, Mifflin County, Oliver Township 1920 PA(sorry forgot to get other information-only Henry & and Sarah were on this census in this location)
The parents and brothers/sisters of Foster Casner are listed below. The parents had a total of thirteen children according to the 1910 census.
Parents: Henry CASNER, born 2-1856,PA Sarah Casner, born 2-1861, PA
Children: Louis E. Casner, born 1876, PA Sam'l H. Casner, born 1878, PA Edward Casner, born 1879, PA Clara Casner, born 1877, PA Ella Casner, born 1873, PA Harry Casner, born 1871, PA Herbert Casner, born 1883, PA John Casner, born 1886, PA Cloyd Casner, born 1889 PA Anna Casner, born 1890, PA Besse Casner, born 1893, PA Foster Casner, born 12-1894, PA Emma Casner, born 10-1899, PA
One note of interest, there were some Stine families living as neighbors on two of the census records.