[8May1993]

兄　（あに） = (My) older brother
弟　（おとうと） = (My) younger brother
姉　（あね） = (My) older sister
妹　（いもうと） = (My) younger sister

when you are talking about your sister/brother you use these words, however you cant use them when talking about someone else brother or sister becuase its not respectful. thats why you use polite form of these verbs

兄　become 　お兄さん　（おにいさん） = (Your) older brother
弟　become 　弟さん　　（おとうとさん） = (Your) younger brother
姉　become 　お姉さん　（おねえさん） = (Your) older sister
妹　become 　妹さん　　（いもうとさん） = (Your) younger sister