Hi David
That was one of my diagnosis along with other RSI symptoms. I don't know of a specific book, but Dr Sarno describes it as an epidemic created in the computer age and sees it as TMS. I think the Great Pain Deception also takes that same view.
For me the same treatment for other TMS symptoms, stop physical treatments, accept that it's TMS 100%, that's been a gradual process for me but I'm there now. Also I've been doing a lot of journaling on repressed emotions, my TMS personality etc.
There are also several RSI recovery stories posted on this site, some who recovered quickly and dramatically, I'm not in that camp it's taking me longer, but the process is working.
Reading any of Dr Sarno's books and substituting RSI symptoms for back pain could be helpful to your sister in law. In some ways we don't have to deal with the diagnosis of bad disks etc so it could be easier to accept.
Take care