Remus Lupin is tired. When one is tired, one makes mistakes. Sirius Black is Sirius Black. When one is Sirius Black, one is highly oblivious and makes huge blundering errors. In amongst all this, Fred and George Weasley might finally meet their idols.
When Sirius fell behind the veil, he disappeared. But where to? To death or to somewhere unimaginable? Things are different, and Sirius has decided that he is there for a reason. But if Peter didn't betray the Potters, who did?