OP, what's the roadblock to getting her into therapy?

If it's affordable, DO that!

And has she ever been diagnosed with ADHD, Autism, OCD, or Anxiety?  Asking, because it's fairly common for those of us with those neurodivergences to have fears & anxiety around driving, and to be "late drivers."

For me (i'm a late Gen-Xer, and discovered that i had both Autism & ADHD in my 40's), the anxiety around driving came out of a childhood awareness of the mass of a vehicle, and an understanding from that of just how much force it took to move--and stop a vehicle. 

Went to the Minnesota Science Museum, where they had a vehicle back then that was crushed into a block, that kids could push around using a special lift.  That crushed car was HEAVY and super hard for a small person like me to move--and that gave me great awareness of tge forces i NEEDED to be paying attention to & in control of, even as a child.

I wasn't aware of my AuDHD back then, obviously.  But i KNEW that i didn't "have what it takes" to drive safely.

I didn't feel comfortable at all behind the wheel, when i was 15 and took "Driver's Training" classes with my peers. The cars we drove for those classes WERE nice & small, and they weren't bad to drive!  

But there was SO much "to pay attention to" when driving--plus the vehicles i had access to were a 1978 Impala (land yacht with bench seats, and i could barely see over the steering whel!), and a mid 80's station wagon--that also had terrible sight lines.

When i was 21, my college friends made me get my Driver's license, so that i could go out with them.  And one had the same type of car we'd had innmy Driver's Training classes--so i was comfortable with the size & sight lines of the car. 

That was the only reason i was able to take & pass the test.

I still didn't drive much, until i turned 23--most of my driving was late-night, driving home from the bar where my best friend & i had gone dancing on Fridays & Saturdays.  

And that late-night driving in a small car was what got me comfortable behind the wheel.

Because, in hindsight, there were "less distractions"--because the darkness meant i was able to focus on the road & the areas close to it.  I trained myself to "bounce" or "sweep" my eyes from one side of the road to the other (watching for deer).

And i learned how to occasionally be checking my driver's side, passenger-side, and rearview mirror, as my eyes kept sweeping the road ahead of the car.

Because there were rarely other cars on the road, and it was dark, there weren't many distractions--which helped a ton!  It made me comfortable with the "mechanics" of driving--paying attention to my speed, the way the car was handling, getting familiar with the lights (and turning the brights on & off!), using the windshield washer fluid & wipers when i needed, just "by feel" and not needing to look where they were, etc.

There IS a LOT of "physical stuff" a person needs to be able to do, simultaneously with the skill of "watching the road and anticipating other drivers."

And a good driver also needs to know their limitations, too!

As an AuDHDer--even before i knew i had Autism or ADHD, i had realized that i CAN'T safely answer calls if i'm driving!  I just can't safely split my attention like that.

So when i drive, my ringer is OFF, and my phone is face-down and just charging if it isn't in my pocket, out of sight entirely.

And the radio station i typically listen to is NPR (MPR, technically, since i live in Minnesota). Or a softly-playing "country oldies" station or CD, if the weather is bad and i need to focus more (because those songs have a slow tempo & i know the lyrics to most of them--so my brain is "relaxed" as i drive).

OP, if your stepdaughter has ADHD, Anxiety, or another neurodivergence?

Explain it to her as, "Your mom and i NEED you to learn to get comfortable behind the wheel, because your Mom NEEDS you to be able to drive her to medical appointments!  Tell her that it's just not feasible as a "one-driver" family, and that you may also need assistance making it to appointments in the future.

It's a safety issue--for EVERYONE in your hpusehold, that needs to be addressed & fixed--and the solution is her being able to drive.

Then?  

Get her that therapy, get a dependable older sedan that has a driver's seat that is comfortable for her and fits her size (for example, probably not a modern-day Impala, because they're still made for large, tall men!), make sure it's not too small or too light either--because very tiny cars can be incredibly difficult to control in windy conditions.

And then help her to get OUT DRIVING in that car--on routes she'd need to take, late at night, or very early in the mornings--when there are few cars on the road! 

Get her comfortable at night, when there aren't as many distractions!  And have her drive the roads she'll need first--so she develops that "muscle memory" for where the stop signs & stoplights are.

Once she has those roads down?  Shift to some "Midday drives" on them, and then eventually "rush hour drives" so she's comfortable in ALL the traffic configurations.

And make sure she gets out and drives in rain & snowy conditions (if you live where it snows!), and learns how the vehicle handles in ice, too (go out to some wide-open parking lots close to home).

She'll be fine onve she gets some pratice in!  But it takes being aware of soooo many things, to be a safe driver, and that can be anxiety-inducing, for those of us who are neurodivergent, and very aware of allllll the various sensory inputs we face every day.

Also, get her some sunglasses to keep in the car, which have narrow frames & narrow arms, to block excess light! (I prefer "black-tinted" ones, rather than brown, but sone folks like other colors), and make sure she KNOWS that she can pull down the "sun visors" and move them to block excess light, too!