I've seen quite a few older people in my city riding around on 3 wheeled bikes, similar to the ones shown here .
They offer quite a bit of stability, without making you low to the ground like a recumbent.
However, I can't think of how you would get one into a car.
Getting a regular bike into a car is hard enough, have you thought about a roof rack?
You almost might want to look into stabilizer wheels which can be attached to any bike, and you may be able to find some that are removable for transportation.
bicycles.stackexchange.com/users/163/Kibbee
Check out the Hase Pino .
Tandem Bike, but built to let a recumbent rider get a full view from the front, without having to steer or balance, while the captain can sit upright with the same awesome view.
There's accessories for seatbelts or handcranks, if that's your thing.
Also, the newest models fold in the middle, so they can fit in a trunk.
On the other hand, mine doesn't fold, but it's short enough that I can just put it on a trunk rack.
bicycles.stackexchange.com/users/1180/Kyle Schmidt
Your best bet is probably a recumbent trike.
Of course this does bring up the transportation issue.
You could look at getting a roof rack or rear platform for the car which could negate the transportation issue.
Another thought would be a tandem.
If you can balance enough to keep you both up and she can manage not to fall off.
Again this brings up transportation as you'd have to have a big car (truck really) to get a tandem inside.
Really I'd look at the recumbent and roof racks.
bicycles.stackexchange.com/users/30/curtismchale
You really should read Shelton Brown's site: http://www.sheldonbrown.com/
He was a great resource for the bike community (RIP) and you have some common threads with him.
He talks about tandeming, which would be an option for you.
A note on the bike having to fit in the car, it is possible to get racks for the tandem, it doesn't have to be transported in the car.
www.sheldonbrown.com/tandem/index.html
The other option that Shelton Brown can also speak to is the trike.
He had one before he died and has an write-up on it.
www.sheldonbrown.com/org/greenspeed/ Cheers,Chris
bicycles.stackexchange.com/users/496/CLJ
I found a long list here (I don't know how good or recent it is, but it is quite long, and pan-national): Where to Find A Four-Wheeled Bike or Tricycle .
The various sellers might tell you whether/how it's possible to transport them by car.
There are also second-level links, for example one of the links is to http://www.roman-road.co.uk/ which then links to http://www.tricycleassociation.org.uk/
bicycles.stackexchange.com/users/1097/ChrisW
My sister works in Cardiff and one of her part time jobs is working with severally mentally disabled adults.
She often takes them out on specially adapted bicycles/tricycles which in Cardiff there are a lot of trails and tracks to enable them to get around.
Although this does not directly meet your requirements of fitting into your van they do offer other sort of mobility bikes which you can read more about the kind of bikes that are provided here: http://www.cardiffpedalpower.org/
bicycles.stackexchange.com/users/2105/Malachi
My wife has Cerebral Palsy (CP) so can’t balance on a normal bike, what are the options so she can go cycling with me?  We don't own a van, the size of any "bike" is important as it will have to fit in a car.
My wife has Cerebral Palsy (CP) so can’t balance on a normal bike, what are the options so she can go cycling with me?
bicycles.stackexchange.com
<safety><tricycle><transport-by-car>
bicycles.stackexchange.com/questions/512
bicycles.stackexchange.com/users/132/Ian
Tandem bikes, recumbents, trikes, the Hase Pino and recumbent trikes are options for partners who cannot ride a two wheel bike. A roof rack would probably be needed for transporting these. A good option might be a Tandem bike, such as Hase Pino. Tandem bikes are easy to balance and allow full view. They are accompanied by accessories, while certain models fold and can be transferred in a car. You might also want to consider a recumbent trike. In this context, a way to solve the transportation issue would be to add a roof rack on your car.
A way to solve the transportation issue would be to add a roof rack on your car. A good option might be a Tandem bike, such as Hase Pino. Tandem bikes are easy to balance and allow full view. They are accompanied by accessories, while certain models fold and can be transferred in a car.  You might want to consider a recumbent trike.