What does it mean for a girl younger than herself to make you call her sister as a joke?

If you are not much different in age, she wants you to call her sister. It means that she thinks she wants to take care of you and thinks you are cute or something. This is a woman's protective nature, that is, she likes you. Although it's just a joke. You can also pander to her and say, "ok, sister, give me some pocket money."