This is my younger brother - please look after him.
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Humble/literary. The 舍 prefix ('my humble') is part of a small set used to refer modestly to one's own family - 舍弟 (younger brother), 舍妹 (younger sister), 舍亲 (relatives). Compare the honorific 令 prefix for the listener's family: 令兄, 令弟, 令妹.
usage
Sounds dated in casual speech today. You'll see it in formal letters, polite introductions, and historical fiction. In daily conversation, just say 我弟弟.
