Paris + Carcassonne + ?

If you include a day trip to Versailles, make it 5 nights in Paris.
Then, I'd take the train to Nîmes for 4-5 nights, and use Nîmes as a base to see Uzès, Pont du Gard, Arles, Aigues-Mortes. Nîmes is a manageable city, and the surrounding countryside is beautiful.
Then on to Carcassonne, where 1 night is enough but 2 can be more relaxing.
And finally Toulouse for 2-3 nights, with Albi as a possible half-day or day trip.

Ideally, fly out of Toulouse back to the US (likely via Paris).

Musee Toulouse-Lautrec is in Albi, the artist's home town. The 13th Century Building was originally the Bishop Palace of Albi Cathedral. It is an UNESCO World Heritage Site. Albi the town is very picturesque.

My wife and I don't like big cities either and similar age group.
We did a more extensive French trip but this was our first couple of weeks.
Flew into Paris, arriving a day and a half after leaving home in regional Australia so pretty knackered on arrival. Quiet afternoon at Lux. Gardens. Our little old hotel was a fair way out in the 14th.
Next day metro into the tourist hotspots just a walk around to have a look.
Next morning took trains to Carcassonne. Enjoyed the 320 kph train. We stayed in a nice little hotel near the station initially. We later moved to a cosy BNB at the upper end of our budget.
Loved Carcassonne. I think many people "do" Carcassonne wrong. Many just go to the Cite, stay there and skip the town all together.
The town is actually not particularly touristed so was a great place to settle into a French trip. We visited the Cite in the evening, in the early morning and during "business hours", it was busy by lunchtime. We found a quiet spot for a nice picnic lunch besides those amazing walls.
There are plenty of things in the town of interest but no big name sights. If you like relaxed exploring then it is ideal.

From Carcassonne we travelled to Collioure.
Collioure was a bit more touristed than Carcassonne when we were there, should be less so April/May.
Lovely little town with a number of beaches and interesting sights. Nice spots to just sit and sip too.
We stayed in an affordable well located hotel.

From Collioure we travelled back to Perpignan then onto the small walled town of Villefranche de Conflent. We totally blew budget on first night staying in a very posh BNB (the old Post Office). Very nice. Subsequent nights were well under budget in a wickedly quirky but somewhat neglected hotel outside the walls.
Loved this little town. Gorgeous location and plenty of points of interest. Above the town is Vauban Fort (as there is above Collioure). This one can be reached by a series of tunnelled stairwells (the 1000 steps).
Walking distance from town there are several large cave systems that can be visited.
There were quite a few day tourists on the Saturday but the town was very quiet in the evening.

From there we caught the iconic Little Yellow Train that dawdles through the Pyrenees to Latour de Carol over a number of hours. We sat in an open topped carriage and were presented with spectacular scenery (at a pretty slow pace).
From Latour de Carol you can pick up the Barcelona Toulouse train which rolls through magnificent countryside.

Toulouse is a pretty big city and we only spent a night there as we were heading much further afield.

Not sure whether that's helpful but if you are after any specific info let me know.

I like both suggestions but will note I haven't yet been to Nimes.

I liked Toulouse a lot. It has some very interesting sights, and the pink-brick architecture is different from what you'll see elsewhere in France. Although it's not a small city, the historic area didn't feel overwhelming to me. And Albi makes a great day trip.

It would be good to have some assurance from locals or frequent visitors that the mistral will not be a major concern at the time of your trip.

By April, spring should be in full swing and the Mistral/tramontane winds are no longer as annoying as they can be in the winter. No problem at all.
And I consider early May to be one of the best times of the year for SE France.

I have a different take based on what you've written. It seems like your sister wants to see what people think of as typical France, the one from paintings, stories, etc. The southeast, where I happen to live and wouldn't live anywhere else, is different from the stereotype. It's very nice, and I've been to every place suggested, including the Pyrenees, but rather than making it the focus, I'd put it at the end.

I suggest going down the west side of France. From Paris, go to the Loire Valley for some chateaux, gardens, and countryside. After that continue to Bordeaux and include a day trip to vineyards. After Bordeaux, it's two hours to the Dordogne region, the Vezère Valley, countryside where many of the prehistoric painted caves are located. Small towns and countryside here, too. From there, it's two hours to Toulouse, and one more hour to Carcassonne. You can fly back to Paris from Toulouse to catch your flight to the US, or you could go to Barcelona on a high-speed train to fly home from there. (One hour to Narbonne where you change to a high-speed train and then fewer than three hours to Barcelona) Either one, you need to be in the city of your flight the night before.
Bonne route.