The best thing you can do for your sister is to be supportive. If you parents are not aware of her eating disorder you need to make them aware before it is to late and the disorder does irreversible damage to her.
This disease kills in a horrible manner as the body to starts to feed on itself. When it does different organs within the body start to shut down.
This is not something that can be overcome just by parents yelling at her to eat. If they force her to eat she will not only be anorexic but most likely become bulimic as well. She needs professional help from a Board Certified Psychiatrist and most likely a Psychologist as well.
Your part is to be supportive and to help her have a better self image. People who are anorexic generally feel they are over weight or have body image problems.
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