Per Album of Genealogy and Biography, Cook County, 4th ed. (Calumet Press, 1896), p. 505 (the reference is at https://www.familysearch.org/tree/person/sources/K8X2-KQZ), the wife of the Peter Schaefer who died 12 June 1894 (in his 90th year) was Helena (Lena) Bleser, who died 1891 at age 79 (thus born about 1812). Per this source, their oldest son was Maternus Schaefer, born 26 August 1833 in what then was Prussia.
Here at familysearch.org is the birth record for Maternus Schaefer, born 26 August 1833 to Peter and Helena Schaefer in Mayen, in what was then the Rhine Province of Prussia. Per this record, Peter Schaefer married Helena Blaeser at St Clement church in Mayen on 5 February 1833 (note that Maternus was born less than 7 months later).
According to this profile at familysearch.org, Helena was born 9 March 1813 in Kürrenberg, which is about 2 miles northwest of downtown Mayen; this was the same place Peter had been born in 1805. Unfortunately, the parish records for Kürrenberg do not seem to have been indexed at familysearch.org or ancestry.com, so I cannot find actual documentation for this.