cousin - son of uncle or aunt - văr
nephew - son of sister or brother - nepot
step-father - non-biological parent married to one's mother - tata vitreg
step-brother - son of non-biological parent married to one's mother - frate vitreg
twin sister - female offspring produced in the same pregnancy. - sora geamana
mother-in-law - female who has a legal affinity with another by being the mother of the other's spouse - soacra
an only child - single offspring - copil singur la parinti
foster parents - non biological parents who adopted a child - parinti adoptivi
in-laws - parents of one's spouse.- socri
Family member names I know: mother, father, sister, aunt, brother, uncle, grandfather, grandmother, brother-in-law, sister-in-law.