Petunia's protection of Harry (aside to Ancient Magic vs Dark Magic)

[Lisa Cocchiarale]

No: HPFGUIDX 82023

Jen Reese cited:

"While you can still call home the place where your mother's blood 
 dwells, there you cannot be touched or harmed by Voldemort.....your 
 aunt knows this.....She knows that allowing you houseroom may well 
 have kept you alive for the past fifteen years." (DD, OOTP,chap. 37, 
 p. 836)

I must ask (although it may have been addressed elsewhere-I am a newbie here) why 
LV doesn't just kill Petunia. If the blood of Harry's mother no longer "dwells" at Privet 
Drive, wouldn't the ancient magic be negated in this case?

-Lisa