There's nothing official from either the manga or anime.
Anything regarding the Cervello's true nature will only be speculation and can thus be dependent on the viewers' interpretations.
While Shoichi said that the Cervello ended up with the Millefiore at some point (Episode 169: Sky Lion Ver.V (Leone di Cielo Version Vongola), it could be implied that this happened after they gave Byakuran the Mare Rings.
In my opinion, their true nature is similar to that of Kawahira's; they're just administrators. I'd like to assume that since Kawahira gave the Vongola Rings to Giotto, he has somehow given it to the Cervello to administer since he's administering the Arcobaleno pacifiers.