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younger brother; little brother; kid brother
brother-in-law (spouse's younger brother or younger sister's husband)
弟はまだ小学生だよ。
My younger brother is still in elementary school.
弟と一緒にサッカーの練習をした。
I practiced soccer with my younger brother.
弟が大きくなってもう私より背が高い。
My younger brother has grown and is already taller than me.