Polite way to refer to someone else's younger brother. Not used for one's own brother.
弟さんはおいくつですか。
How old is your younger brother?
弟さんによろしくお伝えください。
Please give my regards to your younger brother.
お兄さん refers politely to someone's older brother, while 弟さん refers politely to someone's younger brother.
弟さん means someone's younger brother, the direct opposite of 兄さん, which means someone's older brother.
弟 refers to one's own younger brother, while 弟さん is the polite form used to talk about someone else's younger brother.
Compound of 弟 (おとうと, 'younger brother') and the honorific suffix さん.