This piece really hit home for me. I spent a month interviewing teens and young adults about their digital communication habits and can say that your sister's usage is very representative.
Re email - it's no surprise that 15 year-olds rarely use email, but I'm not sure they're suddenly going to warm up to it when they go to college. I had many college students tell me that they rarely use email socially; they just see it as something associated with school.
Might email be dying? It seems like email is to 15 year-olds what physical mail is to a 30 year-olds — a thing of the past.