OOP: Twelve Grimmauld...a problem

[Diana]

No: HPFGUIDX 63367

It occured to me, after really thinking about certain events in OOP, 
that there is a plot problem in Kreacher's betrayal of Sirius.

We find out from Dumbledore near the end of the book that Kreacher 
had taken Sirius's "get out" literally and had left the house.  The 
only place Kreacher thought to go was to family (that is family that 
Sirius's mother still liked before she died), and so he went to 
Narcissa Malfoy, who is Bellatrix' sister.  According to Dumbledore, 
Kreacher couldn't tell Narcissa where the Order was located because 
only the Secret Keeper (Dumbledore) could reveal the house's 
location.  My question is, since Narcissa's mother was Sirius' 
cousin, wouldn't Narcissa and her sister Bellatrix (the escaped 
Death Eater) both have visited the house sometime in the past in 
order to visit their relatives?  Maybe they weren't close enough to 
visit each other, but I'd be surprised if that were the case.  
Wouldn't they already know where the house is located, Secret Keeper 
or not, because they'd been to it before?  

Anyone have a plausible explaination for this?  

Diana L.