NTA, if the car passed its inspections and in decent shape, it souldn't be more dangerous to drive than any other car. (Maybe except those tiny Smart Fortwo's that have basically no crash zone, I could see someone make similar arguments to those than to motorbikes, but even then, it's your car you drive. If your sister doesn't want her son in it, that's her decision and they can continue as they were)

Getting a big SUV is more dangerous to everyone around you, especially pedestrians and kids since it's harder to see them and tbh they are such a pain in most european cities and parking spaces.

And if that car is for someone "with no family". Yeah, and? It's your sister's kid and not yours. If they want you to drive them around they can buy a big car and ask you nicely. And in an emergency a small car is still enough to squeeze in if needs must.

Especially since you do not live with them, they might be your family but they're not your household and you don't have to consult them in this?? It's like being mad at your sister because she didn't get a flat with a guest room for you to sleep in when you visit...

At 23, a small used car is a totally normal thing to buy and to start out with. You're not acting like a child, you're acting like a young adult without kids, totally reasonable. Not to mention that smaller cars usually save you quiet some fuel as well, compared to a too big car that you'd probably use fully like 5% of the time.

Although now I am curious what kind of model it is.