I don't know if either would be alive. I believe that in a way (possibly a portrait) Dumbledore will be there but Sirius I'm not really sure about. If Sirius is not dead then how did Harry inherit No. 12 Grimmwauld Place. It would be wonderful if Sirius did come back though, I was so happy when he appeared in the 3ird book. But I'm really not sure on the matter, I can't want for Deathly Hallows to come out!