I know the original post is a couple years old, but maybe my info will help others.
I had the first injection to that area of the pseudoarticulation, and it was a major success! I had three of the best weeks in years. Called my doc's office last week to see about getting some muscle relaxers as the injection was wearing off. Doc called me himself to find out what results we had. When I told him the details, he suggested a second one to verify the results. If the second one has the same results, then we will be talking to a surgeon about having the excess bone removed. There is an end in sight to my back pain.
To those who have transitional vertebra with pseudoarticulations, keep pushing your doctors until they take that seriously. I have degenerative changes in the lumbar spine, but no narrowing of the spinal column, etc that would be causing me pain. Injections at the nerve roots near the degeneration had no major affect on my pain level. Injections at the area of the pseudoarticulation had very noticable results. I will keep posting regarding the process.
Your sister needs to keep pushing the doctors until she finds one that takes things seriously. The pseudoarticulation on the left side definitely should be looked at closer. I have the same issue on the right side, with some of the other issues as well, but not in the same degree.
I finally found a doctor that takes the pseudoarticulation seriously. It has taken me over 4 years. So many doctors push this off as a normal occurrence that is not suppose to cause back pain. If it is only one sided, and the partial sacralization has not fused to the sacrum (which is my problem) then it can definitely be a source of the symptoms you are describing.
Your sister should also slow down, limit her activities, and give her back a chance to rest. I have found that bending, squatting and kneeling to be the biggest causes of flare ups. Driving is also a contributor to my flare ups.