How to tell someone they suck at driving

Oppo, I have a problem. A certain relative mine, who I very often find myself riding in cars with, is an absolutely terrible driver. I've driven cars on tracks and at autocrosses for a few years, spent weeks on small boats in the open ocean, been on planes more times that I can count, and I NEVER felt even the slightest hint of motion sickness. Never, that is, until I met this woman. I guess the most obvious two issues I have are with her braking and steering.

First off, I have NO IDEA how someone who has been driving for 30+ fucking years can not know how to modulate brakes. She treats the pedal like a switch, it's either full or no braking with little grey area. Coming up to a stop nearly always involves several hard braking and release cycles.

The other issue (and I think the one that contributes mostly to the motion sickness), is with her steering. She is absolutely INCAPABLE of holding a line in a corner. Instead she drifts from one edge of the lane and back again as if trying to divide every corner into as many small sharp turns as possible. By the way she also keeps her hands at 11 and 1 o'clock positions, so that might have something to do with it.

She has a few more shitty driving habits (poor merging, left lane sitting) but they fall more into things that annoy rather than actually scaring me. I think the worst part is she is CONVINCED of her driving prowess and that everyone around her is terrible. I have personally been in the car with her while hitting curbs and medians at over 20 mph because she "didn't see them", and more panic stop situations than I can list. Despite this, she claims that she has never been in an accident with another car that was her fault, but I think that is mostly due to people just now wanting to deal with arguing with her (she has one of the most severe cases of entitlement I've ever seen).

Basically, do I just suck it up and not say anything? Or can someone think of a gentle way of saying "I think you are a terrible driver, you are dangerous, and I feel dread at the mere thought of getting in a car with you"?