The contact's sister owns a 2021 Tesla Model Y. The contact stated that while her sister was driving on the highway(speed unknown) with the autopilot engaged, an 18-wheeler drifted into her lane and hit her vehicle. Due to the impact, her sister's vehicle hit another vehicle and bounced back and forth between both vehicles. The vehicle continued to the drive despite the accident and no warning lights illuminated on the instrument panel. The airbags also failed to deploy. A police report was filed. The contact's sister and her mother, the passenger in the vehicle, were taken to the emergency room where they were treated for various body aches and bruising. The whereabouts of the other drivers involved in the accident were unknown. The vehicle was towed to an independent tow yard where it remained in their possession. The dealer nor the manufacturer have yet to be notified of the failure. The vehicle had yet to be repaired or destroyed. The failure mileage was approximately 2,000.