Early forms of 弟 show cord wound in an orderly sequence around a handle or stake. Scholars connect that layered wrapping with the older idea of order or succession; from there the character came to name the younger brother who follows an elder sibling. The exact object varied in ancient forms, so this drawing keeps the well-attested core: cord looping around a central shaft.
A piece of culture
Older or younger? Chinese family words often build relative age into the name. 哥 is an older brother; 弟 is a younger brother. In Mandarin, the familiar form is 弟弟 (dìdi). Cantonese reads the same written form as dai6 dai6, while everyday families may also say 細佬 (sai3 lou2). One word can therefore tell you both the relationship and who came first.