Another one I've found thanks to the fanfic recommendation pages at TVTropes:
Hocus Pocus, Adele Polkiss (Harry Potter). Adele is the seven-year-old sister of Piers Polkiss, one of Dudley Dursley's gang. And through her brother she meets ten-year-old Harry Potter, a curious boy with a mysterious past, and a wonderful secret... Magic! AU, HPxOC, and complete. Rather well written, and a nice ending, plus an "extra scene" afterward.
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.
Hocus Pocus, Adele Polkiss (Harry Potter). Adele is the seven-year-old sister of Piers Polkiss, one of Dudley Dursley's gang. And through her brother she meets ten-year-old Harry Potter, a curious boy with a mysterious past, and a wonderful secret... Magic! AU, HPxOC, and complete. Rather well written, and a nice ending, plus an "extra scene" afterward.
-- Bob
---------
Then the horns kicked in...
...and my shoes began to squeak.