Any suggestions would be much appreciated............................

Adopted two female kitties from the local SPCA in the fall of 2012, they're sisters and great cats.  One problem that is baffling, to say the least;

Charlie Girl has gained A LOT of weight, not so for KoKo Kitty, her sister.  They are fed twice a day, the same food and amounts.  Thought maybe

Charlie was eating KoKo's food, so they are now separated when eating.

I'm an older person and Charlie is so heavy that I can barely lift her!  Talking over 20 lbs.  HELP!!!!!  (her health seems fine, just very overweight).

Welcome to The Cat Site.
Cats have different metabolism, just like humans. I'm sure you know people who can eat anything, and not gain an ounce!
What are you feeding them? Wet food is better than dry food, no-grain preferable. Dry food has more calories in it too.
Fancy Feast Classic (classic only) is a decent brand available at most any grocery store. FFC is low in carbohydrates, which is what you're looking for. Oh, and "diet" pet foods don't always work; they have more carbohydrates, less protein/fat (yes, fat!) than they need. It's kind of like eating a lot of cheese curls and you still feel hungry.
And when was the last time she had a wellness check? Maybe you can get a neighbor or relative to help you take Charlie Girl in.
I assume both are spayed and are indoor cats only.
Here is an article about obesity in cats. As indicated in the article, you should check with your vet before beginning any weight loss program--to rule out any medical issue and to make sure you don't cut the amount of food too quickly.

Usually, wet food or homemade food are the best solution to a cat's obesity...  

Every cats metabolism is different, different cats need different amounts of food.  http://catinfo.org/?link=felineobesity is an excellent resource that talks about obesity in cats.  I have 5 cats and no 2 of them eat the same amount of food daily, each cat has their caloric needs calculated based on activity and size and then are fed accordingly.