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.
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.