It's quite possible for siblings who are biologically related to have different blood types. I'm an A+ and my father was also an A. My mother was an AB and my brother is an O. Even though type O is more common than A, B, or AB it is the recessive gene. Having type O blood means the blood cells don't have the antigens needed to have A, B, or AB blood. I believe that my parents had to be "carriers" of the type O gene in order to produce a child with type O blood since they didn't have type O themselves. That would mean that I carry the type O gene and pass it on to my children-if I had any. If I were to have children they could have type O blood if their father were type O-or at least have the gene for it.
The + gene is the dominant and the - is the recessive. My dad said he forgot whether he was A+ or A- but he knew he had type A blood. I told him either he was an A+ or my mother was an AB+ since I have positive blood myself.