In the Twilight series isn't the treaty between vampires and werewolves already broken?

No because the treaty is only broken if the cullens ever bite a human rather than an animal but (warning spoilers ahead)when Edward wants to save Bella in BREAKING DAWN after she gives birth to Renesmee Edward ask if there could be an acception in the treaty from Jacob (who is his own leader after he breaks away from Sams pack after he wants to plan an atack on the cullens for trying to have a deadly vampire/human baby (Renesme)), for Bellas life.

...well.. i think it already has...in twilight movie in la push... the treaty is only broken if the cullens will bite a human and when the quiluete tribe would reveal the Cullen's secret (for vampires should not reveal their existence)... but Jacob already broke it himself.. he told Bella (in twilight book) about their quiluete legends about the cold ones...specifically the CULLENs...

true...but Jacob mentions that it is a legend and he doesn't even know its true until he becomes a werewolf.