Sirius felt sorry for the owl. He would like to brain Bellatrix with a rock to show her what it felt like, but he knew better than to do that (for once).
"Could you try to be nice? Do you even know how?" he asked, sounding positively astonished by the idea of a nice Bellatrix. Nice Bellatrix was an oxymoron. He knew what those were thanks to Remus.
"You never know. The people you call filth are keen on the whole idea of redemption and forgiveness." Sometimes. Sirius certainly wasn't speaking for himself, but maybe for some other more forgiving chap out there.
"Just because you don't consider me your cousin doesn't mean I can't consider you mine. My blood runs thicker than some patchy tapestry."