Is the accident your fault if you stop suddenly and cause two cars behind to rear-end but do not get hit?

No, it is not your fault if you stop suddenly and cause two cars behind to rear-end but do not get hit. When you stop suddenly, you are not breaking any traffic laws. The drivers behind you are responsible for maintaining a safe following distance and being able to stop in time. If they fail to do so and rear-end you or each other, it is their fault, not yours.