おとうとさん
弟さん
(someone else's) younger brother
Note
[弟さん]…Oさんの「弟さん」、私の「弟」
[otootosan]…is used to refer to someone else's younger brother. "otooto" is used to refer to one's own younger brother.
[弟さん]…Oさんの「弟さん」、私の「弟」
[otootosan]…is used to refer to someone else's younger brother. "otooto" is used to refer to one's own younger brother.