Why does my maltese hate my brother?

Just adopted a 2 year old male maltese a few days ago. He has been well mannered with all the people he has came in contact with @ my home, EXCEPT my brother the same age as me. Hes 20. My maltese did not like him from the moment he met him barking and growling. I feel bad because my bro was so excited to meet him. He is our first dog. The dog also hides for the etire time my brother is around then come out the second he leaves. Im not sure if this is normal behavior.