We've gone over that topic hundreds of times on the thread SPEW, I think it's in The Books section. It's quite interesting. As for Kreacher, as I think has been said before, a house-elf can only be set free if they are given clothes by their master. If all the family are dead, I'm sad to say it, but I guess the elf would just be left in the house to rot. It's shocking
I can say though, that that probably won't be the case with Kreacher. Sirius hasn't got any sisters, that is correct I think, but even cousins are members of the Black family. He was the last of the male Blacks, because all the females have changed their last names (ie. Mafloy & Lestrange.) Kreacher will probably in some way find himself in the hands of Lucius through Narcissa, and thus in the service of evil.
And yes, Siana. I am quite sure myself that Sirius will have left 12 Grimmauld Place to Harry, as well as all his other valued possesions (except maybe a few to Lupin.) Also JKR said that Harry would be having his shortest ever stay at the Dursley's place in Privet Drive, so why do you think that is, and do you think he'll be going to 12 Grimmauld Place, when he leaves early? I doubt it, because that was where he went at the end of Book 5, and he never really goes to the same place when he ;eaves the Dursley's.
Bk1: Straight to the platform on 1st Sep.
Bk2: To the Burrow.
Bk3: To Diagon Alley on the Knight Bus.
BK4: Back to the Burrow.
BK5: 12 Grimmauld Place.
I think he will go back to the burrow again. It makes sense, as he hasn't been there for a while.