Yep...there's no justifiable excuse why they don't have a car seat.
They've lived here long enough, would've seen car seats in your car & other peoples when out driving, shopping centre carparks etc. Been told/given pamphlets about it at the hospital while still pregnant/before being discharged after the baby was born.
It's irresponsible & negligent of them & I couldn't give two sh!ts what their excuse for not having one is. Nor should the fact that they're family stop you doing what's right.
Your responsibility is to that child not their parents. Report them to the police asap. I would!!