a possibility can be the phonebook to find adresses to write persons with the names: http://www.herold.at/en/telefonbuch/
take at states Styria and specially district GRAZ but also other districts can often be a possibility. Graz has 22 hits, no idea if you will write a letter and send in copies to all... http://www.herold.at/en/telefonbuch/steiermark/rucker/
another possibility is to look which parish-church is nearby to the stated birth-adresses, so you can ask also there if they have some more to brothers and sisters for later times. i will look on map if i can find out but takes some time.