I thought of exactly that when I read that line again--wait, how do you get 12 OWLS again? It's kind of neat to think of other kids doing it and sticking with it.

Hmm, Sirius knew Regulus was a DE but did Regulus know Sirius worked for the Order?

I don't know...but then, even if he did there might have been a lot of reasons he wouldn't think he could go to him or maybe didn't want to go to him.