house elves' secrets

[sylviampj]

No: HPFGUIDX 174014

 I read OOTP for the first time fairly recently (I seem to have read 
all the HP books in the wrong order - sigh - at least I noticed the 
foreshadowing!) At the beginning of OOTP Sirius is showing his 
family tapestry to Harry at 12 Grimmauld place and says: ' I see 
Tonks isn't on here. Maybe that's why Kreacher won't take orders 
from her - he's supposed to do whatever anyone in the family asks 
him -'

Tonks is the daughter of Andromeda nee Black, Sirius' cousin.  So if 
Kreacher is supposed to be taking orders from her then he should 
also take orders from her sisters Narcissa and Bellatrix. 

At the end of the book when Dumbledore is explaining to Harry how 
Kreacher betrayed Sirius he says 'He was bound by the enchantments 
of his kind, which is to say that he could not disobey a direct 
order from his master, Sirius. But he gave Narcissa information of 
the sort that is very valuable to Voldemort, yet must have seemed 
much too trivial for Sirius to think of banning him from repeating 
it.'

Any explanation for this seeming inconsistency? Is it a slip on 
JKR's part or have I misunderstood?