Very interestin tidbit I found while checking the chat archives

[rcraigharman at hotmail.com]

No: HPFGUIDX 22106

--- In HPforGrownups at y..., meboriqua at a... wrote:
> I like the idea that perhaps Petunia was the one to blow up Marge,
> or even that Harry and Petunia together blew up Marge.  Vernon is
> the only one who says anything about the incident, by the way. 
> We don't hear a word from Petunia, IIRC.
> 
> Good one!

I would suggest that "IIRC" is no replacement for canon.  Before the
incident, the only thing we see of Petunia is that she and Vernon are
looking tense over what to answer Aunt Marge when she asks what
James did for a living.  On the other hand, Harry is getting angrier
and angrier, and for that there is canonical evidence.  Further, both
Vernon and Petunia are alarmed as Marge blows up and starts to rise.
Harry's "reckless rage" (p. 30) is what drives him to pull out his
wand and point it at Vernon and then run off into the night.

Moreover, as I point out in my other post, Fudge absolves Harry of 
responsibility after the incident.

It may be a rough thing to say, but a lot of speculation is easily
resolved by a glance in the books....

....Craig