Illegitimate Birth Records

I have an older half-sister who was born in Norway, I believe about 1926-1930. We have the same mother, Lillie (Lily) Anna Ellefsen (maiden name). My sister's name is Elizabeth and she was probably born somewhere around Oslo, but I am not sure. Lillie Anna's parents' names were Conrad/Konrad and Otilie (Rosenvinge) Ellefsen. Could someone please tell me how I might go about finding records of my sister's birth or baptism? Also, my sister's husband's name is Allen Lierfall and they live in the Barum area? Would their marriage information be onlinle? Thank you, Louise