Hi VS, it's difficult to answer your question in a way that will help your sister, as the side effects undoubtedly vary according to the treatment area.
The actual process is completely painless. Below are a couple of photos from the net that I meant to post earlier. To ensure that the rays were focused accurately every time, they made a very tightly fitting mould exactly like that shown. Once I was on the table, the whole thing was over and done with in 5 minutes.
The effects take a while to kick in. The first thing that struck me about your post is that your sister is having 15 treatments, whereas mine involved 35. I posted after 17, and things got worse from there. Mostly pain, tiredness and inability to eat. The pain, of course, is from an inflamed throat. I take co-codamol (which causes constipation!). I doubt pain and eating will be a problem for your sis.
The surface area gets sore and tender, similar to sunburn. I haven't found that too much of a problem, though it varies.
I've just finished my course of treatment, and my next review is on 15 Sept., though I have assorted other appointments in the meantime with nutritionists, speech therapists, etc. I'm reliably informed that things will continue to bubble away for a while before I get back to normal.
I've lost 10 kgs so far (no bad thing in itself), despite being being on a high calory diet and assorted supplements. I simply can't get much down. I dream of roast lamb, mint sauce and roast tatties.
The photo below is me and my grandson, who was getting (and giving himself) a round of applause after ringing the bell.
I had a lovely surprise this morning when I opened my door to find two district nurses beaming at me. Night Crawlers, here I come...