Concept for you all (canon au): Sirius wasn't just the first person to find out about Jegulus, he was the ONLY one
And he thought it was so sweet and loved teasing James for it
Until Reg left Jem to join voldy and then he was like 'I know he's my brother and all but he's just a dude, men ain't shit'
Point is- he kept the secret forever
But that didn't stop him from making jokes about it up until the day he died
For example:
*during OOTP*
Sirius: *sneaking out of Reg's room giggling*
Harry: what are you doing?
Sirius: nothing!
*during deathly hallows*
Hermione: why is there a gryffindor tie with JFP embroidered onto in in regulus' room?
Harry: what??? Those are my dad's initials
Sirius in the afterlife: 🤠I'm so funny
Or
*james and Lily's wedding reception*
Sirius: I am so glad for the happy couple. Especially James, my best friend, my one and only, my brother- wait no, that would make 5th to 6th year very strange.
Everyone thinking James and Sirius had some sort of affair: ???
Sirius: Ah yes, that's where I was. Ahem, Lily-Pad, my favourite carrot-top, I remember the day you and James met-