I don't know the reason why you need to give your sister's Social Security Number to your accountant. But I do know that you should ask her permission. Identity theft is a major problem today, and being careful with giving out a personal Social Security Number is one of the ways to prevent identity theft. I would say that it is probably illegal for you to give out your sister's Social Security Number. But even if it wasn't illegal, it is just more ethical and respectful to ask for her permission first.

Are you a lawyer?
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