Was your sister's car Keyless Entry? Relay devices are commonly used across the country to steal vehicles. Essentially, you have two thieves - each with a relay device. One stands at the door of the car and one near the house. The device receives a signal from the car key inside and transmits it to the second device next to the car. The car's systems are then tricked into thinking the key is present and it unlocks, before the ignition can be started. The thieves don't actually have the key, so they don't turn the car off because they then can't turn it back on. Usually, cars are driven straight to shipping containers or warehouses to sell on abroad. Here's a video you can watch of it happening: www.honestjohn.co.uk/news/miscellaneous/2017-11/wa.../

You can buy a Faraday bag, which blocks signal being transmitted. You can also invest in an immobiliser system, which is far more expensive, or a steering wheel lock like Disklok or Stoplock to stop the car being driven off.