Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sōbrīnus, i, m., and sōbrīna, ae, f. [contr. for sororinus from soror, and therefore prop. an appellation for the children of sisters; hence, in gen.], a cousin-german, cousin by the mother’s side, Fest. p. 297 Müll.; Dig. 38, 10, 3; Just. Inst. 3, 6.