The word "brother" by itself does not tell us whether it means 哥哥 or 弟弟. We have to say either ''older brother" or ''younger brother." In informal speech we can also say "big brother" (哥哥) or ''little brother" (弟弟). It's the same with ''sister."

If someone says "I have a brother", you will not know if he is older or younger unless they add more information. You could ask, "is your brother older or younger than you"? They could have said, "I have a brother who is younger than I am." or "I have an older brother". You might call him your "big brother". If a brother is younger than you, he would be called "a little brother", or a brother younger than I am.

there is no way only if one says " i have a younger brother" or " i have a elder brother" or "i have a brother, he is younger than me" and ...

Surprisingly, it's perfectly normal in western culture to say you have a brother/sister, without needing to specify whether he/she is older than you. If we ever need to be specific, we add "older/younger" to "brother/sister".

ask him/her