Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
mātrŭēlis, is, m. [mater], a mother’s brother’s son, a cousin-german, first-cousin on the mother’s side (cf. patruelis, a father’s brother’s son; post class.), Dig. 48, 9, 1; Vict. de Orig. Gentis Rom. 13, 8.