Of course, one could say almost any technological advance is also cheating, but at least anyone who has the technology can use it; magic is not nearly that fair. :-)

The sisters thing has always been my 'hook' into Petunia, and I confess that I based a lot of her reactions to Lily on a massive exaggeration of my occasional sense that my own little sister completely outshines me. (Fortunately, we are not estranged!) So I am glad that my take on a sibling relationship worked for you.