Care and Feeding is Slate’s parenting advice column. Have a question for Care and Feeding? Submit it here .

Dear Care and Feeding,

My wife “Jane” and I have two girls, “Clarissa,” 3, and “Brianna,” 6. Generally, they get along well, but Clarissa bugs her sister sometimes. When Clarissa recently took apart a Lego set Brianna had been working on for the better part of a week, she angrily said that she wished her sister would disappear. My mother-in-law happened to be over when Brianna said it, and told her that if she wished for her sister to disappear, invisible fairies would hear her and take Clarissa away forever. Now, Brianna is scared that Clarissa will be spirited away by imaginary pixies and is constantly checking on her and is even afraid to go to school because she worri

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