弟【おとうと】
younger brother, little brother, kid brother, brother-in-law (spouse's younger brother or younger sister's husband), pupil, apprentice
兄弟【きょうだい】
siblings, brothers and sisters, brothers, siblings-in-law, brothers-in-law, sisters-in-law, mate, friend
弟子【でし】
pupil, disciple, adherent, follower, apprentice, young person, teacher's student-helper
子弟【してい】
children, sons, children and younger brothers, young people
義弟【ぎてい】
brother-in-law (spouse's younger brother or younger sister's husband), younger stepbrother
師弟【してい】
teacher and student
弟妹【ていまい】
younger brother and sister
弟子入り【でしいり】
becoming a pupil (of), becoming an apprentice