A polite term for someone else's younger brother (honorific).
田中さんの弟さんは大学生だと聞きました。
I heard Mr. Tanaka's younger brother is a university student.
失礼ですが、弟さんのお名前を教えていただけますか？
Excuse me, but could you tell me your younger brother's name?
弟さんはどちらにお勤めですか？
Where does your younger brother work?