Hello aarchi!
Welcome to the Chronic Pain forum!
I'm going to cut to the chase here, and say that Dr's don't do surgery when they do not know what is going on with a severe back problem. You state that the Dr is unsure as to what the problem is, and suspects "inflammation" or that the discs are "moving"? ....and that the only thing they can do is surgery?.....if her Dr has stated this, then she she should run as fast as she can away from this Dr!....and seek a competent physician who can better tell what the real problem is.
Normally test are performed such as MRI's and CT scans, and such, in order to better diagnose the problem. Again, these can give the DR's a better picture as to what may be going on with the spine.
If these test have been performed, and there is indeed a problem with the spine, ....then, there are several options one can choose, before selecting corrective surgery. Those options can include "Physical Therapy", "Epidurals"...and several others that slip my mind here at 3:30am
Surgery should be the last resort, but not rejected out of fear for the unknown. The other option, would be to seek out a pain management Dr, to help her through the pain issues, but please do realize that this is only a mask on the real problem.
Again the bottom line here is that if her Dr is unsure as you have stated here, and then is suggesting surgery...then something is very wrong! She needs to seek a second opinion. Although I commend you seeking advice on behalf of your sister...I'm afraid that the information you have provided us is just too vague....and the members here would need more information....so that we might be able to point your sister in a better direction.
We are not Dr's here, but we can certainly share our own personal experiences, and maybe suggest a better path for your sister to seek.
Again "Welcome"....and see if your sister can, join us here....we would be happy to visit with her!
SE