Hmm. Actually, if they pretend to be friendly, none of the werewolves has talked, and Aro hasn't arrived yet, they might be able to dupe the Volturri into thinking that they were sent there by the Cullen coven to look into something going down on their territory, so as to get close enough to the siblings to be able to sneak attack and disable or kill them and allow the werewolves to polish off the rest.