The literal sense of uterin is ‘of the womb’ – in this case ‘brothers from the same womb’. Curiously the Latin substantival root, uterus, has no equivalent in Anglo-Norman or Middle French.
a.
frere uterin
1kinshiphalf-brother (who has the same mother, but a different father):
(
c.1334; MS: s.xivm
)
Ely, q'estoit le frere Jacob uterin de part sa mere
114.2
uterin (c.1334)