Slytherin and Sirius Black

[roseswicegood]

No: HPFGUIDX 82022

OK--don't take the wands out--if this has been done, my only excuse 
is I checked the FAQs and OotP FAQ too, before I posted, so here 
goes. 

I guess I don't get it.  It seems logical that Wormtail and Snape 
were Slytherin, and James, Griffyndor.  What about Sirius?  He was 
thrown out of his home at sixteen, but why?  Because he was put in 
Griffyndor?  It's a strange combination:  James, Sirius and Peter. 
 
I'd appreciate your comments.

Rose Swicegood