Wilhelm Halvorsen's Children

I am looking for any living relatives of Wilhelm L. Halvorsen. Address in 1958: Gladengen 5. Oslo. Worked in the prison of Oslo. I think he was in the resistance movement during the war, he was 44 year at this time. Wilhelm was born in 1895. He had 3 siblings. Alpha, Albert, and Hjordis. His parents were Henrik and Inge. Henrik's sister was Albertine. She is my great grandmother. Does anyone have any ideas on other ways to search for living relatives? I've sent a letter to all the Halvorsens in Oslo with no luck. Elizabeth Harvey