I really enjoyed this fic. I think Sirius is a very interesting character, particularly in relation to his family vs the family he made for himself (ie the Marauders). I think you've really shown the man underneath the confidence and arrogance here, a man who perhaps wishes he could have had a loving family, or at least that he could have kept his brother. I liked the fact that Sirius felt guilty about Regulus, and felt that he should have been able to do something, that's a very typical sibling reaction.
I liked how you mingled past scenes with present scenes. Particularly the first two past scenes were fantastic.“I understand Reggie, I do, I just learned to pretend that I’m perfect for them, so will you. We’ve got to make them think we think we’re good enough and maybe they can’t see our flaws.” That's such a beautiful line.
You also showed a great sense of war in this fic, particularly in Sirius' distrust of Remus, who has been a close friend for years. James' injuries added to this, too. There was just a sense of things falling apart and dying, which was well done.
Just a few nitpicks - I thought the boldened words interrupted the flow a bit. I know they were important words, but it sort of detracted a bit for me.
Also, you misspelt Kreacher and Apparated (you wrote Kretcher and Apperated).
One more thing, which is that you talking about the Death Eater as "it" just seemed a bit strange to me. I suppose you were trying to animalise/objectify the cruelty of Death Eaters, but that doesn't really make sense to me, because Dumbledore values all human life, and I think that's what he would have taught the OotP. If you used it because of gender ambiguity, I think the only female Death Eater ever mentioned in canon is Bella (we know for example that Narcissa isn't one, though she sympathises with them), so it would be natural for Sirius to assume that the Death Eater was male.
Anyway, those were just a few small things which didn't quite feel right in this story, otherwise I really enjoyed it. I hope you submit more stories soon.
Oh, one last thing, I think you characterised Dumbledore very well, he felt very believable. As did Lily.