There are many ways to sprain an ankle. The two most common are turning the ankle in or out. Some people twist the foot in, causing damage to the ligaments on the outside of the ankle. Others land on the inside of the foot, tearing or partially tearing a different set of ligaments.
By using cadavers (human bodies preserved for study), scientists have been able to study the foot and ankle. By moving the cadaver ankles in different directions, it is possible to see which surrounding ligaments are stretched. When a force is applied that is strong enough to tear the ligaments, the ankle is said to be “sprained.”
It is likely that you and your brother twisted your ankles in different ways. This could account for the different locations of pain and symptoms.