I imagine that Regulus and Sirius haven't really mended their relationship after Hogwarts but the person to bring them back together is Y/n.
It starts small when Remus and Sirius finally adopt her. She's a tiny cute baby and their nights were filled with constant crying and fussing. So one day Regulus sends over a package. Sirius didn't know it was from him until he opened it to see his childhood stuffed toy and Regulus's old blanket. With a note saying:
"I still remember those nights where you'd lend me Mr. Snuffles because I was terrified of mother and father's arguing. I'm glad that dear Y/n gets to have parents like you and Lupin. I know it's been years but I hope you're happy with your new family.
-R.A.B"
Then once he gave the two items to Y/n. Their nights were no longer sleepless and were actually peaceful.
