A handshake is a globally widespread, brief greeting or parting tradition in which two people grasp one of each other's like hands, in most cases accompanied by a brief up-and-down movement of the grasped hands. Using the right hand is generally considered proper etiquette. Customs surrounding handshakes are specific to cultures.
It is not allowed to deal with him as a real physical brother. Hijab is must on you in front of him if he is Baaligh and it is Haraam to shake hands with him because he is not your real physical brother.
I tend to smile, put my hand to my chest, and say 'Sorry, I don't shake.' By placing your hand on your chest, you are offering an alternative way of greeting, which makes greeting less awkward. Another method is to get a sympathetic colleague - if you have one - to inform people ahead of meeting you that you don't shake hands with the opposite gender.