In addition to the proteins (agglutinogens) existing on your red blood cells, other genes make proteins called agglutinins (a-GLOO-tin-ins) that circulate in your blood plasma. Agglutinins are responsible for ensuring that only the blood cells of your blood type exist in your body.
Older children and teens are unlikely to start calling a stepmom mom or mother partly because of their fear of disloyalty to their biological mom. It is vital for all parties involved -- biological parents and stepmom -- to support their child's choice of what to call the stepmom.
A stepbrother is your stepfather's or stepmother's son through a previous relationship. For instance, if your father married a woman who already had a son or if your mother married a man who already had a son, then that boy would be your stepbrother. However, in order to have a half-brother, you must have either the same mother or father as he does. This means that if your father or mother remarried and had a son, than that boy would be your half-brother.