Well to start, unfortunately,I have no idea where you might find a new car for $12,000.
For two elderly occupants and space for a wheelchair I suggest a hatchback rather than a sedan. Most hatches have more space than a trunk and a lower, flatloading point making loadingand unloadingconsiderably easier.
Your next primary concern seems to be ease of entry.For that either aSubaru Forester or a KiaSoul would nicely fit the bill. The Subie will cost more but has the added benefit of AWD if you mustdrive in the snow.
Concerning fuel economy, if95% of your daily travels are lessthan 90 miles (70 miles ifthetempsdrop below 45 degrees).I suggest you seriously consider an electric car.If you can get over the somewhat funky look of an electric car,the Nissan Leaf or Kia Soul electric versionswould fiit the bill nicely.In additon to meeting your stated criteria, they are very economicalto run and maintain, comfortable, and arevery reliabable. If you need todrivefurther than 90 milesa few times each year, simply rent a carfor such a trip.
Used electric lease returnsare plentiful in many major cities, tend to have 15,000 to 25,000 miles, normally are in excellent shape and cost$9,000 -$13,000.
If the electric simply won't work for you,my suggested order of preference would bethe gas versionKia Soul, Honda Fit (outstandingfuel andreliablility,albeit a bit loud and bumpy - get the Base modelrather than Sport for a smoother ride)or SubaruForester.