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1) No she will cope with more food and possibly alcohol.
2) Even if she wants to change she will have no idea what to do: what to eat, how little to eat, how to exercise, how to dress, how to talk to men. These are the only life skills you need as a woman.
3) She needs to get the habit in when she's young. Otherwise it will be a lot harder to change when she gets older.
4) IMO you should take your sister to the gym and make up a food plan.
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