Hello~Has she had any x-rays, CT Scans or an MRI. I would think that without some form of imaging, a diagnosis can not be confirmed. To me, it sounds like some type of nerve impingement.
I also would suggest seeing a chiropractor. He/she will take x-rays of the lower back, study them and then go over the results with you. If it is something they can help, they will start treatments, in a few weeks, the issue should start getting better.
Many healthcare providers will start with an EMG (electromyography) and an NCS (nerve conduction study). They are often done at the same time. The results can help differentiate between a muscular problem and nerve problem. A lot can be ruled out with these simple tests. Based on your sister's clinical findings advanced medical imaging such as x-ray and MRI may be suggested.