Maybe you are worried for nothing, but research has shown that in many cases parents don't use the safety seats correctly. In recent years there are positive developments in this area - standards like ISOFIX and LATCH are appearing and they improve things significantly. What they do is use different belts to attach the seat to the car, instead of the normal safety belts.

What car does your sister have? The newer the car, the higher the chances it is equipeed with ISOFIX or LATCH.