Does the certificate list the place of death? Where did Margaret live?What does the Census say about her? If she immigrated in 1881, she may have come over with my John, in which case we might presume they are related.If she settled on Billy Buck Hill or anywhere near St. Michael's, the presumption could be quite strong.
Were any of the Diethorn children married at St. Michael's?If so, you can get a transcript of the marriage record from the archivist at the Diocese of Pittsburgh.This might reveal Mr. Diethorn's first name and possibly the names of some witnesses (who may also be named Sommer). You may also be able to find a birth or christening record for Mary.
I'm sorry, I don't have any information on the Diethorns other than the Edward Diethorn family who lived on St. Paul Street.