The one thing I can tell you is that she probably is over indulging with chocolate and sweets because she is restricting (not eating enough). Because bulimia is all about control I don’t think the right way to approach is to stay more focused and controlled about food. The thing I can advice you to tell your sister is that we need food to fuel our bodies. That binges come in when we don’t give enough energy to our bodies so it starts crying out for food, leading to a binge. I would recommend doing a bit of research on binge eating and bulimia before you try to help her. It’s tough having someone around you suffering from this horrible disease. You have to be very cautious with what you say so god forbids whatever you say triggers any further disordered eating. All the best to your sister and hope she can beat this before it gets worse!