I receive infusions every eight weeks in the chemo room, although, thankfully, I don't have cancer. I've been going to the same place for the last five years and I do see that a lot of the cancer patients are just plain cold! They have hospital blankets in the room, and although they do the job, it is a lot nicer when they have something a bit more cuddly and cozy. I would think your sister-in-law would appreciate a lap size quilt, maybe with its own bag so she can easily bring it to her chemo. Most patients I see have a ton of appointments to manage, along with blood work results, and medication names, and I think it would be helpful to have some pockets in the bag and maybe even a pocket appointment book or some sort of notebook along with a pen.
If you're stuck on a design, you should think about what you might have made her before she got this diagnosis. She's still who she is, even with cancer.
I'll keep her in my thoughts.