What's the difference between a sister and a younger sister?

Generally speaking, it depends on habits and local dialects. Generally, in the north, it's called Laomei [Laomei Er], but it's seldom called Sister. If she asks you to call her Xiaomei, it means that your is not particularly friendly and should not be very familiar. Generally speaking, it gives people a more intimate feeling to be called Laomei than Xiaomei. Xiaomei is usually a polite title compared with a girl younger than her, so you should know about the current communication between the two of you before calling!