Disclaimer: I don't own Harry Potter. Wish I did but. :)
A prelude and set up to the following story…
Sirius Black did not give baby Harry Potter to Hadgid after Godrics Hollow was destroyed and as such did not run off on a stupid revenge trip to kill Peter Pettigrew.
Instead Sirius took Harry to the only people he knew he could trust at that moment, his cousin Andromeda Tonks nee' Black and her muggle born husband Ted Tonks.
Meanwhile, without Sirius hot on his tiny rat tail, Peter went back to the Death Eater Headquarters and safe house that the Voldemort's Inner Circle had gathered. Peter knew that in time the truth would be revealed (that he had been the secret keepr and the one who revealed said secret to the Dark Lord) and so he used the Inner Circle's confusion at the lost connection to their master to do his dirty work for him.
At this time only the order of the Phoenix knew Voldemort was gone (or at least as much gone as he seemed to be). Peter convinced the Inner Circle that Voldemort was not entirely gone and had simply used powerful magic's to transcend into the realm of magic after he had defeated his prophesied adversary (which was sort of true and was in essence a goal he had been working on) and that he had given a final order before disappearing into the Ether.
That the Inner Circle needed to hunt down and kill Sirius Black.
Believing that the Dark Lord would be displeased if they failed him when he returned. The Inner Circle began the hunt for Sirius. It did not take long for them to track his magical signature to the muggle neighbourhood that the Tonks' dwelled. The Death Eaters surrounded the house and attacked.
Sirius however had prepared for the attack but had not truly anticipated the numbers of Death Eaters. The Inner Circle had thrown all they had at the task.
Wards deflected most of the initial barrage, but under the sheer stream of spell fire the wards broke, tearing the house apart down to the foundations.
It was a mad fight from beginning to end. Sirius confronted Peter in the midst of battle and killed the man before he could change and escape.
The Tonks' however were no match for seasoned battle mages. Using all their might they pushed Sirius, Harry and their only daughter into the fireplace sending them away from the fight via floo.
The last vision that the young Nymphadora Tonks had of her parents was of them falling to a blinding green flash, as the fireplace collapsed sealing their escape.
Dumbledore and the Order of the Phoenix arrived just as the little muggle house exploded. Death Eaters scurried into the darkness disapparating in echoing pops.
No trace of any survivors remained and the Order concluded all were lost.
And so we begin this fascinating and titillating tale of AU fantasy. Enjoy!