Child safety seats

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. 

Yeah, I'm pretty sure the Renault would have ISOFIX.

You aren't worried for nothing, a LOT of seats simply don't provide enough structural support for the seat to fit snugly with the normal seatbelt whilst the vehicle is moving. 

If clips don't come with the carseat, you should definitely look into extra safety clips, see the final tip from my recent article on this very subject: 'Choosing A Child Car Safety Seat: 4 Tips Every Parent Should Know'- there's a product called SeatSnug that can provide the extra safety these seats need which solves the problem immediately, but makes sure that the kid is as comfortable as normal.  Definitely look into getting that extra bit of kit for the car journeys you make!