This is an interesting spoiler…
I think the Charming family can be ruled out–Regina is the ememy, they hate her, she hates (with the exception of Henry) all of them.
I think it is safe to rule out Red Riding Hood. She has shown herself to be truly loyal to Snow White and Prince Charming. To have her (or Granny) betray everyone would be very out of character. The only way it would work is if it were explained enough to believe that Red would come to that choice. And even then, it would be hard to believe that she would side with Regina. 🙂
Belle (I think) won’t betray anyone, either. True she may be darker in season 2, but I don’t think it will be to the point of where she will betray everyone. She deeply loves Rumpelstiltskin, and it makes no sense that she would help the woman who imprisoned her–unless Regina was somehow able to control her without her knowledge. 😥
August has been shown to not be trustworthy (nor dependable at times, though I still love him) so there is more of a possibility. I still doubt it–he has his father on his mind, and I think he does feel incredible guilt for what he did to Emma (as he should) so for him to side with Regina does seem a reach, though it could happen if Regina were to promise him something in return.
I can’t picture The Blue Fairy. No matter how much damage she had done, she still has helped in the battle against Regina. It wouldn’t make sense for her to suddenly change sides–though we have seen her to do what she feels is best for everyone. She may feel the need to turn on everyone for what she deems to be the greater good.
But Hook could be the one that turns people against one another–they are building him up to be a smart villain. He could be the one to trick one of the characters.