Sirius and Regulus Black, the youngest children in the Black family, have one secret they'd shared with nobody, what goes on at home. They were good at hiding it, Sirius by being happy and cheerful, Regulus by being mopey, though that was just so he could cry and nobody would suspect a thing. It worked for a while until one of Regulus' house mates, Severus Snape, caught on to the act, he had caught Regulus writing a panicked note to his mother the evening after he got it, something Regulus never did.
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