Best Answer - Chosen by Voters
Children are much more adept at seeing spirits for two reasons. One, children are much closer to the spirit world--they left it quite recently to be born. Two, children don't know that things like spirits don't exist. No one has told them yet that ghosts aren't real. So yes, it makes perfect sense that your friend's little sister would see a spirit.
I saw spirits when I was little--I just didn't know that at the time. I was with my grandpa, and we had gone over to see a friend of his on our way to the store. I was outside by my grandpa's truck when a man came by on a bicycle. The man on the bicycle looked to be about 20 years old, and he was dressed as if he was back in the 1930's. He had on the hat that was sort of poofed up in the back but mooshed down in front over the brim. He had on the sort of pants that buckled at the knees, with dark socks leading to his old-looking shoes. He had on a white shirt with an argyle-style sweater vest on. His bicycle looked old too. It wasn't a bicycle with one big front wheel and a little bitty back wheel, but it was obviously an old bicycle. Everything around me was brightly colored, like the green trees and blue flowers, but this man and his bicycle were sort of washed out. Everything about him was shades of brown and gray. He stopped his bike for a minute, looked at me, tipped his hat, said, "Cheerio!" in an English accent, and rode away. I turned around to see if my grandpa had seen him, but he hadn't. When I looked again to see if the man was still down the street, he had vanished. I told my grandpa what I saw, but he thought I was making it up. I was about 4 at the time.