Several parts combine into one character.

弟 originated as a pictograph of a leather strap wound around in sequence — coiled in order from one end to another. The original meaning was "sequence / order / what comes next." From this concrete image of orderly winding grew two semantic streams: (1) the family-hierarchy sense — among siblings, the one who comes "later in sequence" is the younger brother; and (2) the abstract sense of order itself — preserved in 次第 (cì-dì, "sequential order") and 弟子 (dì-zǐ, "the one who comes later in learning" = disciple / student of a master). Two related meaning-clusters from one root image.

Korean reading "je." 兄弟 (hyeongje, brothers / siblings — central family vocabulary), 弟子 (jeja, disciple / pupil — important in Confucian teaching tradition: "Confucius's disciples"), 徒弟 (doje, apprentice — used in trade-craft contexts), 義兄弟 (uihyeongje, sworn brothers — the Confucian-romantic ideal of friendship treated as kinship; central trope in Korean historical drama and literature). Korean keeps both senses (brother and order/disciple) active.

Mandarin dì, 4th tone. 弟弟 (dìdi, "younger-younger brother" — the doubled form is the everyday Mandarin word for younger brother), 兄弟 (xiōngdì, brothers), 徒弟 (túdì, apprentice / disciple), 弟兄 (dìxiōng, "younger-elder" = brothers — note inverted compound has same meaning). The Chinese reduplicated 弟弟 contrasts with the formal 兄弟 in everyday speech.

Japanese has multiple readings reflecting different historical layers. テイ (tei) — 兄弟 (kyōdai, siblings), 義弟 (gitei, brother-in-law). ダイ (dai) — 弟子 (deshi, disciple / pupil — used in martial arts, traditional crafts, and academic mentor relationships: 弟子入り deshi-iri "to enter discipleship"). Kun-reading おとうと (otōto, younger brother) — 弟 (otōto). Japanese family vocabulary distinguishes elder-versus-younger siblings strictly: 兄 (ani, elder brother) / 弟 (otōto, younger brother), 姉 (ane, elder sister) / 妹 (imōto, younger sister). The hierarchical distinction is mandatory in everyday Japanese in a way it is not in modern English.

Memory aid: a leather strap wound in sequence — the "later in order" position became the younger brother; the abstract "sequence" sense became 次第 and 弟子.