ei søster · sister
Feminine (ei)
Indefinite singular
ei søster
Definite singular
søstera
Indefinite plural
søstre
Definite plural
søstrene
Feminine nouns (ei) can always be written with en as well: ei bok, boka or en bok, boken. Both are correct Bokmål.
How to guess the gender Nouns: gender and articles