Hi, cold is a viral infection that spreads through erosol means by air. yes you are right what you caught from cold may be by your brother s cold. It is impossible for humans to get immuned to this cold because every time different type virus causes infection. The good thing is it affects to nose and throat mainly by running nose, congestion, blocked nose,throat pain,cough which mostly subsides by itself. If not then u can take symptomatic treatment available OTC along with steam inhalation,saline nebulizer,avoid cold products and u will be alright.