The financial and environmental implications of your sister's habit, particularly the wastefulness of unused food, appears to present a valid concern...such an entrenched position would be irritating to your sister. If you threaten people's dignity by moralising at them, you will not persuade them. This is a lesson we have all got to learn.
I don't think your dilemma is about food waste. If you are really worried about the environment, why not start with the enormous amount of food thrown away by supermarkets after the end...
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