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/

maybe you also ask at registry-office in Graz if they give details to the brothers and sisters you found which may have descendants. normally document protection(Privacy Policy) for a longer time in Austria, but maybe they do when you send an attachment at e-mail which proofs the relationship - maybe also send the answer of diocesan-archive. i have no idea if they do and how much it would cost, but make attempt and ask if possible and how much would have to pay if they do. civilrecords started 1938:
http://www.graz.at/cms/beitrag/10017599/313159/

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.