What about US documents on the children of the 2 ancestors?
....grandfather (John) .. and his siblings (Charles, Mary, Steve, Peter (Pat).....
I would think that the church baptismal record would have parents place of birth as well as mother's maiden name.
If your ancestors married in US than the church marriage record and state marriage application should have the place of birth.
Were you able to find male ancestor's draft registration form? Were either of the ancestor's naturalized citizen?