From Old Danish systær, søstær, from Proto-Norse ᛊᚹᛖᛊᛏᚨᚱ (swestar), from Proto-Germanic *swestēr, cognate with German Schwester and English sister (probably influenced by Old Norse). The Germanic noun goes back to Proto-Indo-European *swésōr (“sister”), cognate with Latin soror and Russian сестра́ (sestrá).
søster c (singular definite søsteren, plural indefinite søstre)
From Old Norse systir (“sister”), from Proto-Norse ᛊᚹᛖᛊᛏᚨᚱ (swestar), from Proto-Germanic *swestēr, from Proto-Indo-European *swésōr (“sister”).
søster m or f (definite singular søstera or søstra or søsteren, indefinite plural søstre or søstrer, definite plural søstrene)
From Old Norse systir (“sister”), from Proto-Norse ᛊᚹᛖᛊᛏᚨᚱ (swestar), from Proto-Germanic *swestēr, from Proto-Indo-European *swésōr (“sister”). Akin to English sister.
søster f (definite singular søstera, indefinite plural søstrer, definite plural søstrene)