I guess the only thing you can do to help her is to be really mean. Don't buy any junk food. Make your whole family go on a healthy diet. Talk to her. I know that she is a teenager and will 'roll' her eyes at you if you talk to her about her weight, but do it anyway. Its really important. Also take up maybe a yoga class. I've heard there very good for you. Maybe once a month have a day when you just eat bad food, but after that day you eat really healthy? Its good you have realised she needs some help. Hope this reply has helped!