Excellent story. I love how spare it is, how the meaning is carried in the exquisitely-chosen details, how alive you bring Petunia (and Vernon, Dudley, Piers). Your explanation of Petunia's treatment of Harry rings so true that I think it will become my personal canon.
The structure builds and grows just like the cement and the geranium. It's like ...I don't know, a magic beanstalk or something, flowing and twisting and carrying the reader with it to the perfectly-inevitable closing that is its own new world. It's a story in which "nothing" happens -- and yet everything does (everything I need to know, at any rate).