Sounds like she has been carried a lot of the time or the parents have been down on the floor with her so all she had to do was scoot between them.

Put her in the exersaucer or something similar every day or even a few times a day to get the legs moving. Another thing to work on balance is to use a large exercise ball and have her sit on it, lay on it etc while you hold her hands. She will increase her sense of balance doing that.

When she falls over instead of picking her up assist her body through the motions of getting up. If she can't push herself up then her arm strength is poor as well. Doing things with the arms overhead such as holding scarves above her head for her to reach and bat at will help increase shoulder/arm strength and work on balance too as she will be looking up and need to rely on her body to stay stable not the horizon.