Alice's power of viewing the future is subjective, ie, her visions change if the person changes his mind/decisions/actions.
It will be more clear to you if you see the climax of Breaking Dawn Part 2.
In the climax scene, Auro clutches Alice's hands & then suddenly yanks her away to his men; the fight begins & ends with Carlisle, Esme dead & so do the other 2 Volturi heads & Edward & Bella behead Auro. The scene that cuts to the before the fight & we realize that the fight was happening in Alice's visions. Alice than say something like "You know the future now. You have to choose" to Auro. And Auro changes his mind. So the future also changes.
Volturi is, in its essence, a Monarchy in the vampire world. Although there are Marcus & Caius on the dais, the real power is in Auro's hands.
The Cullens are a reputed & respected families in the vampire world especially because of Carlisle's benevolence.
Edward & Alice weren't willing to join the Volturi given their methods of governance. The only way for Auro to change their mind (wink ) was
to force them to join against their wishes.
This can be accomplished if Carlisle's head was on the edge of a Volturi's sword.
The Volturi, with their army could easily over-power the Cullens.
In the answer up till here, I've ignored the fact that Alice can see the future. So considering this, if Auro decides to move against Cullens in order to force Alice & Edward to join them, then Alice can see the future & the Cullens can either flee or gather other vampires to join them against Volturi like in Breaking Dawn.
The Volturi doesn't have the element of surprise against Cullens. So they cannot plan anything themselves unless they can find a loophole in
vampire laws (exposure) to move against Cullens which they did in Breaking Dawn.
Another way out for Volturi was if someone else could take out the Cullens like in Eclipse. In Eclipse, it was a surprise attack because
Alice didn't know who was making decisions. The newborn vampire army was an exposure threat but the Volturi kept mum so that that army could take out the Cullens & then Volturi would come to the rescue & save only Alice & Edward.