The story
- Old Norse · derived fromsystirin
- Middle English · inherited fromsustren
- English todaysistren
From Middle English sustren, plural of suster influenced by Old Norse systirin, plural of systir; the Old English plurals (of sweostor) were ġesweostor, ġesweostra, and ġesweostru. Analysable as sister + -en (plural ending).
Etymology from Wiktionary, CC BY-SA 4.0.