What should I do? [Mature Topic]?

Confront your sister and tell her you glanced at the message. Ask her why she wants to mess up her life by having a baby that young. Tell her to think about how your family is doing and that you can hardly afford bills. Tell her to think about that and also that she doesn't want to have a baby that young of an age because then she won't be able to enjoy herself like a normal teenage girl.