• 兄弟きょうだい — siblings / brothers
• 弟子でし — disciple / pupil
• 実弟じってい — biological younger brother
• 義弟ぎてい — younger brother-in-law
• 弟分おとうとぶん — junior / younger companion
• 師弟してい — teacher and student

弟 originally showed a hand tying a bundle, symbolizing order or position — later representing a younger male in a group or family.

It’s used for siblings and for someone in a junior role, like a disciple or mentee.