Sirius and Remus both thought that the other was the spy that betrayed James and Lily. Remus thought Sirius was the spy because of the prank he played on Snape when he told him to press the knot in the Whomping Willow leading him to a werewolf Remus.
Sirius guessed that Remus was the spy because he thought that Peter wouldn't have the guts to join Voldemort. He used Remus's lycanthropy against him saying that Remus must be the spy because he's a werewolf and 'untrustworthy'.
A/N: I'm writing a oneshot for the next chapter! It's probably going to be up in a day or two. I'm not a super quick writer unfortunately. Thanks for reading!
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Some Siriusly Obvious (or not) Harry Potter Headcanons
FanfictionWhere I write some thoughts, theories, headcanons, and oneshots all about the Wizarding World. Started 8/13/20 Does include profanity
