I have an "a" blood type, my brothers and sisters have the same type "o" blood type, is that possible?

Yes. Yes it is. Your blood type a is because on of your parents was type a and the other type o. Your siblings happened to get the recessive o allele from the type a parent and o from the other. You got the a allele from the a parent making you a. It's totally possible.
Yes. Yes, that is possible. A is dominant so it is possible for your siblings to be type o with the same mother and father.