OOP: Phineas Nigellus and his future role in the series

[bowlwoman]

No: HPFGUIDX 66252

I posted something along these lines earlier when I posted 
how "second cousins once removed" works, but I wanted to put this 
theory out in a separate post.

Sirius and Arthur are second cousins once removed.  This means at 
some point between the two of them they have a great-grandfather and 
a great-great-grandfather in common.  Because Arthur is the elder of 
the two, for this theory I assume he is in the older generation.

Arthur and Sirius had a relative in common quite a ways back who 
would be Arthur's great-grandfather and Sirius' great-great-
grandfather.  Sirius mentions that Phineas Nigellus is his great-
great-grandfather.  Even though Sirius would have 16 great-great 
grandfathers, the fact that JKR mentioned both Phineas' relationship 
to Sirius and the Sirius/Arthur relationship makes me wonder if 
Phineas isn't the common ancestor.  That would make Phineas Arthur's 
great-grandfather as well.  

This might help explain all the Weasley boys being prefects/HB.  They 
have a birthright from a previous headmaster, albeit one who was 
quite strict and disliked (sounds like Percy).  Due to the fact that 
the Weasleys are considered blood traitors, they've either forgotten 
or refuse to acknowledge their own ancestors (as the rest of the 
family refuses to acknowledge them).  Even though Sirius was pretty 
much disowned from the Blacks, Phineas was still upset when he found 
out Sirius was dead.

I'm sure Dumbledore is aware of this, so he should have Phineas play 
an even greater role in the last two books, seeing as the Weasley 
family is now entrenched in the revived Order and Ron is Harry's best 
friend.  

I would like to see Phineas used as a spy on his family in the future 
books.  Since there are no Blacks left, the painting at 12 Grimmauld 
place may go to either Bella or Narcissa.  That would be an 
invaluable tool to have for espionage and finding out what the enemy 
is up to.

Any thoughts?

bowlwoman