It's a bit of a grey area, and the final answer could depend on the law in your state. As guardian, your brother makes medical and living decisions for your sister. This can include sharing information with others. HIPPA is really irrelevant because the individual is free to share their own medical information, and your brother, in that regard, stands in for your sister.

However, in most states, the guardian is supposed to carry out the wishes of the person under guardianship to the extent those wishes can be determined. This is known as “substituted judgment.” If this is the standard used in your sister's state, then it sounds like your brother should not disclose any information to your parents.

Other states follow a “best interest” standard, under which your brother could disclose information if he determined doing so was in your sister's best interest. Even here, the consideration should only be what is in your sister's best interest, not that of your parents.