Having neither a family car, nor the wherewithall to acquire one of my own, I didn't bother getting my driver's license at 16, as I could have. I was 22 when I took lessons, got my license, and bought my first car. It was a big deal when I did it, but it wasn't a big deal to wait.
And for you and your niece and sister? Well, I'd ask the same question as sofiecushion. If they give her a car, that's a recipe for disaster, IMO -- I don't care HOW responsible she seems to be, it seems to bring out the worst in kids -- and if she's not responsible...
But other than that, I'm inclined to think that having stated your concern, you've done what you can. The upside of being the auntie is much the same as for being the grandparent -- you get to enjoy the kids, without having the day-to-day grind of bringing them up. Well, the downside of both is that there comes a point when, no matter how concerned you are, having said your piece, you have to back off and let the parents make the decisions -- and hope there won't be an occasion when you need to be there to support them as a result of a bad one.
Pray lots and try to be even more alert than usual to opportunities to encourage responsibility in the girl. More than that can only serve to make bad feelings with your sister IMO.
What are the licensing laws where she lives? In BC, we have a graduated licensing programme. You don't start the process until you are 16, and the first two stages, Learner and Novice, have restrictions. It takes three years to get a full license. Nothing's perfect, but it sure does load the dice in favour of safety/responsibility.