弟: younger brother | 悌: to do one’s duty as a younger brother

弟 dì

: younger brother; junior male
: I (modest word in letter)

悌 [variant: 弟] tì

: to do one’s duty as a younger brother

弟 | おと oto

noun
: (Humble (kenjougo) language) younger brother

弟 | おとうと otouto

noun
: (Humble (kenjougo) language) younger brother

弟弟 di4di: younger bro

弟子 di4zi3: pupil; disciple; follower [ていし teishi]

弟兄 di4xiong OR 兄弟 / xiong1di4: bros; comrade
[兄弟 | けいてい keitei OR きょうだい kyoudai]

弟妹 di4mei4: younger siblings [ていまい teimai]

兄妹 xiong1mei4: brother(s) & sister(s)
[けいまい keimai: older bro-younger sis OR きょうだい kyoudai] ご兄弟 | ごきょうだい gokyoudai: siblings; brothers and sisters

徒弟 tu2di4: apprentice; disciple [とてい totei]

表弟 biao3di4: younger male cousin (maternal)

舍弟 she4di4: younger bro [しゃてい shatei]

悌友 ti4you3: to show brotherly love for friends
OR 友悌 you3ti4: brotherly bond

悌睦 ti4mu4: to live at peace as brothers

孝悌 xiao4ti4: filial piety and fraternal duty
[こうてい koutei: filial piety; brotherly love]

愷[恺]悌 kai3ti4: happy and easygoing; friendly

Japanese common word | expression | idiom
弟姫 | おとひめ otohime
: youngest princess

四海兄弟 | しかいけいてい shikai keitei
: people in the whole world being all brothers; universal brotherhood

兄弟牆に鬩ぐ | けいていかきにせめぐ keitei kaki ni semegu
: sibling rivalry; quarrel among friends

Chinese expression | idiom
難兄難弟 nan4xiong1 nan4di4
: brothers in hardship. Fellow sufferers (in the same boat)

親兄弟，明算帳 [亲兄弟，明算帐] qin1 xiong1 di4 , ming2 suan4 zhang4
: even with your own brother, keep clear accounts

孝悌忠信 xiao4 ti4 zhong1 xin4
: Confucian moral injunctions of fidelity; piety to one’s parents, respect to one’s older brother, loyalty to one’s monarch, faith to one’s male friends