I like my brother's friend what should I do?

You should probably do as you would with any other guy your friend likes. Except, in this case, he is your brother, so there would be boundaries in talking about him to her. He has some rights to privacy (personal home activities, thoughts, etc.) and you should not cross those. She is your friend, and she deserves the same. Otherwise, you could get entangled in the relationship and lose closeness with both.