Whats The Name Of This Disease?

As a child, I knew 2 boys around my same age group who were brothers. I was always told they were sick, but when we were young they seemed healthy. We would run and play like normal children. As we grew older they couldn't walk or anything anymore. They were both put into wheelchairs and as the years went by it seemed like they would get weaker. I was told that this disease was only active in boys but the girls carried the trait. It was hereditary. They had an older sister but she was healthy. I was told if she ever had kids who were boys, they too would have the disease. One of the older boys died at the age of 18, and the younger one died when he was 20. People with this disease don't live for very long. I really hope someone can help me. Im totally ignorant about any kind of information on this. I can only remember small details family would tell me.