the denali clan seem to be a sort of safe harbor for the cullens: edward fled from forks to alaska to stay with them with the intention of staying with them forever, to be far away from bella but as we all know, he changed his mind. and when the cullens had to leave forks due to edward's decision they all met up with the denali clan during the spring break, which is where alice had her vision of bella "committing suicide".
if the tension with the werewolves and the cullens nullifies the treaty, putting bella in the middle of a possible war-like atmosphere, its possible that one of the cullens(alice, most likely) will go to the denali sisters taking bella to protect her from the animosity that is surrounding her humanity. since the denali clan are the only vampires within the cullens' distance that choose to follow their own diet, its reasonable why they are so much close to one another. i see the denali sisters, as well as carmen and eleazar, as the other version of the cullens, but a bit older and much closer and flamboyant(the denali sisters love their humen men). they are constantly mentioned and i love to know more of them, since i believe close friends are family as well, probably more so.
as far as the volturi are concerned, my opinion is that if bella is not killed(like jane and caius want) it will be because of aro and marcus. aro is fascinated by bella's private mind and marcus is in his own way fascinated by the relationship between bella and edward, possibly with her relationship with all of the cullens. if she is turned by the volturi, it will not bode well since aro and marcus would probably want to keep her for themselves. they are the essence of their kind, as alice so bluntly tells jake before the flight to italy. if she is turned by one of the volturi, edward would truly lose her since she would become the sort of vampire he is not; one that is ruthless and cares nothing for humanity as edward and his family do. victoria would probably kill bella, since edward killed her mate. she would probably do what james wanted to do in the first place, and torture her to settle the score. i doubt victoria holds bella's humanity in any form of regard; her plan is to do what edward did.
"She pined in thought, and with a green and yellow melancholy she sat like patience on a monument, smiling at grief" -Viola, Twelfth Night