Polite way to refer to someone else's younger brother. Not used for one's own brother.
How old is your younger brother?
Please give my regards to your 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.