Well, sure. They suspected that should the Volturi force them to join, she would start brain-frying them into liking their superiors and want to please them. It was later revealed she would do this to any talented vampires the Volturi wanted that they came across, while breaking their ties to outsiders. But this was mentioned maybe once or twice out all four books. Eleazar and Carmen leave the Volturi unhindered, which isn't really the case in this story. I suspect the choice they were given here was to have severe restrictions on their freedom or be completely altered by Chelsea. I think she may be more powerful in this story, but like I said it wasn't really discussed in canon.