This is fabulous. The moment I saw the inspired pairing, I knew I would have to read the fic. Aberforth and Sirius make a lot of sense - and it fits into my personal canon that Sirius was lurking by the Hog's Head when pretending to be a lovable stray. I've got a lot of love for Aberforth, and I love how he takes care of Sirius here - and yet stays true to himself and does not give in when the call for the Order comes. His past regrets and resentments ring very true, as well.
Your Sirius is perfect. Damaged and traumatised, yet able to interect in such a human and affectionate way with people (and hippogriffs) he cares for. And the description if Sirius changing into the new robe is very sexy indeed.