No magic outside school

[mmemalkin]

No: HPFGUIDX 57024

Sorry if this has already been discussed...
Petunia Dursley grew up with Lily, and presumably knew that she 
wasn't allowed to do magic outside of school.  (Although I'm sure 
Lily was very clever, I doubt she could keep the secret for seven 
years.)
Why did Petunia never share this information with Vernon or Dudley?
Vernon forbid Harry to do any magic, and maybe she's content to let 
her husband believe he's in charge of the situation.  But she knows 
Harry is teasing Dudley. (US CoS p. 9-10) It seems odd that she 
wouldn't put a stop to that with a few words to Dudley.
It's possible she thinks the ban isn't important because she's so 
afraid Harry will use magic anyway, but even if she thought Harry 
would just ignore the ban, it seems like she would have told her 
family how he was *supposed* to behave.
Any ideas?

~Diane  who's signing off to do some of the chores Petunia made 
Harry do that day...