Remus/Tonks
I've been trying to justify it for a year, trying to understand why Sirius would do it. And I've had a lot of time to do it, being largely isolated and unemployed. Plenty of free time to try and hate him.
But — it's almost harder to believe that Peter did it. And hard to believe that Peter would do it again. I know he had some ... resentment, sometimes, about James, and Sirius to an extent. But to be so in service to the Dark Lord that he'd actively bring about his return?