Welcome to Just Answer and thanks for your question. If one parent has A- blood and the other parent has AB+ blood, then the possibilities for the children are: A+, A-, B+, B-, AB+, or AB-. Having scarlet fever as a child does not affect fertility. It is mumps that may or may not affect male fertility. A sperm test ordered by your doctor would determine if you are fertile or not.
In order to tell if your O+ male is your brother, I would need to know the blood type of either one of your parents.