Disclaimer: It's very simple. If you can find it (the character, place, thing, or idea in question) in a book with J.K. Rowling's name on the cover, it's hers. If not, it's mine. Thank you very much.
Chapter 1: Parallels
"Mr. and Mrs. Dursley, of Number Four Privet Drive, were proud to say that they were perfectly normal, thank you very much."
It had been true thirty-odd years ago, and it was still true, though the Mr. and Mrs. Dursley who now lived at Number Four Privet Drive were not the same couple who had lived there then. In some respects, though, the families were very similar. The main one was that neither the first or the second generation of Dursleys to live in the house could stand anyone thinking they might be odd or unusual in any way at all.
Vernon and Petunia Dursley had only had one child, their son Dudley, now grown, married, and master of the house. They had now moved to a retirement community, where Vernon played golf and cards and Petunia gossiped and played bingo. However, there had been another child in the household. When Dudley was a year old, his cousin, Petunia's sister's son, had been left on their doorstep, with a letter explaining that he was now an orphan and their responsibility.
For reasons Dudley had never really fathomed, his parents had taken the boy in, and he had lived with them for ten years, until he left for some freak school Dudley could never remember the name of. But something strange was always happening around him... and quite a few times, those strange things had involved Dudley personally...
Oh, Dudley Dursley knew the hazards of having a wizard in the house personally. And he was determined not to let it happen to him. When his wife Marcie had become pregnant, he had worried for nine months. What if the child turned out to be a freak, a wizard or a witch? It had happened to his grandparents.
It turned out that there was more than one child, that Marcie was pregnant with twins, and of course Vernon and Petunia had come to see their grandchildren. They had cooed over little Chester, saying he looked just like his daddy, but one look at Helen and Petunia had gasped "Lily!" and fainted dead away. No explanation was ever forthcoming.
Dudley knew, of course, that his aunt's name had been Lily, and what she had done and been, although it didn't usually bear thinking about. Marcie knew everything as well – he had confessed it all to her before they were married, and she had assured him that she could love him anyway. So that day, the parents of Helen Dursley made a pact that they would keep their daughter under careful control. If she showed any signs of unnatural or even just unusual behavior, they would discipline her immediately and strictly.
Dudley was determined that he would succeed where his parents had failed. Harry Potter might have turned out a wizard, but Helen Dursley would never – never – be a witch.