弟 began as a picture of a cord wound around a stake in regular order, turn by turn. From 'order / sequence' it came to mean the younger brother — the one who comes LATER in the birth order. It's a cousin of 第 (dì, sequence/ordinal).
弟弟 (dìdi) is a younger brother. 兄弟 (xiōngdì, older+younger brother) means 'brothers' and, more loosely, close male friends — '我们是兄弟' ('we're brothers') is a strong statement of loyalty. 弟子 (dìzǐ, 'younger one') means a disciple or student of a master.
A cord wound in order, turn by turn — the one who comes later: younger BROTHER.
兄弟 ('brothers') also means close buddies — calling a friend 兄弟 says 'you're family'. And 弟子 (disciple) shows the same idea: a younger one who follows a master.