I suggest getting William's death certificate from the PA Dept of Health. It should list his parents. His obituary may have it too as well a surviving siblings. Check with the Historical Society of Western Pennsylvania and the Western Pennsylvania Genealogical Society to see if they can help you with the obit. It will help to know the exact date of death. If you don't know it call the cemetery office. They should be able to at least give you a date of burial.
William and Anna were married ca 1891. If Anna was from Allegheny County there's a good chance they got the license at the Allegheny County Register of Wills. Be sure to ask for a copy of the application which might have data on the parents.