Good evening, @mmiles24. I am a recent recipient of a heavy-duty course of neuropathic itch. If she feels as bad as I did, then it is urgent that your SIL be given relief from this horrible sensation. Here is the link to the oatmeal bath:
https://www.webmd.com/skin-problems-and-treatments/eczema/colloidal-oatmeal-baths
And you probably already know that warm water is better than hot? I have used an Rx topical like Triamcinolone Acetonide Cream for mild cases of "the itch". My recent "itch" attack was so severe that my PCP and Dermatologist are now collaborating to find out what was different. I can tell them....."agony".

Another tip.....:
As I literally crumbled to the floor, my life partner just began rubbing me all over.......with a medium pressure. That was a big help.

The most effective treatment was a small jar of what is called Releaf Balm. It is one of the products that I also use to manage my neuropathy pain. At this point, I truly feel that the two sensations, pain, and itch.....are equally torturous.

If your SIL lives in a legal cannabis state you may elect to try the topical. I have always managed my neuropathy pain with medical cannabis so I won't know of opioid pain products.

May you and your SIL be free of suffering and the causes of suffering.
Chris