broþor

Old English

brōþor m

The word brōþor is attested in many different forms in the plural. The unmarked form, identical to the singular, is “attested early,” according to the Oxford English Dictionary. The Oxford English Dictionary also describes the forms with <-ru> as “frequent,” along with the forms with <-ra>. The forms with <ġe-> are described by the Oxford English Dictionary as being from “a typical West Germanic formation in which the prefix denotes mutual relationship.”
The genitive singular breþer is occasionally found, either patterned after the masculine consonant stems before they had adopted -es in this context or by analogy with the rare genitive singular dehter (“daughter's”), in turn modelled on the feminine consonant stems. In Northumbrian or late Old English, analogy with the a-stems can instead yield brōþres, although unmarked brother is still common in Middle English.
Analogy with the consonant stems also rarely results in the umlauted nominative/accusative plural brōeþre in Mercian.^[1] In late Old English, this type spreads into West Saxon as brēðre; it is very common in Middle English, perhaps due to the influence of Old Norse brǿðr.^[2]

	singular	plural
nominative	brōþor	brōþor, ġebrōþor, brōþra, ġebrōþra, brōþru, ġebrōþru
accusative	brōþor	brōþor, ġebrōþor, brōþra, ġebrōþra, brōþru, ġebrōþru
genitive	brōþor	brōþra, ġebrōþra
dative	brēþer	brōþrum, ġebrōþrum

^ Hogg, Richard; Fulk, R. D. (1992), A Grammar of Old English, volume 1: Phonology, Oxford: Blackwell, →ISBN, page 132
^ Dance, Richard; Pons-Sanz, Sara; Schorn, Brittany (2019), “breþer n.(pl.)”, in The Gersum Project ⁠‎^[1], University of Cambridge, University of Cardiff, and the University of Sheffield.