also: てい
younger brother; little brother
Ordinary family term for a male sibling younger than the reference person. As a standalone word it is normally read おとうと; てい is not the usual everyday reading.
See also: 兄
My younger brother is a university student from this year.
younger brother-in-law
Only with reading おとうと
Used for certain in-law relationships: one's spouse's younger brother, or one's younger sister's husband. For explicitness, 義弟 is often clearer.
See also: 義弟
妻の弟が週末に泊まりに来ます。
My wife's younger brother is coming to stay this weekend.
Only with reading てい
Archaic use read てい. In modern Japanese, 弟子 is the normal word for 'pupil' or 'apprentice.'
古い文献では、「弟」が弟子や見習いを指すことがある。
In old texts, 弟 (tei) can refer to a pupil or apprentice.
He has two younger brothers.
He is my husband's younger brother.
I care for him like a younger brother.
younger daughter
younger brother's wife; sister-in-law (younger brother's spouse)
Kana spelling is practical in casual, learner-facing, or child-oriented writing, though the kanji 弟 is very common.