Hi Tammy
I also have a bad back and although my DS was an independent sleeper from a young age there are always hiccups in the routine when it goes off track due to developmental leaps, sleep need changes etc so there is often a need to support LO through these tricky times and having a bad back makes it even harder.
If your DN was previously sleeping independently then I would think PUPD is not necessary - often LOs can return to independent sleep when the developmental leap has passed and if/when the routine is suitable (it may need a change) by using a firm hand (the equivalent to shush/pat but it can be adapted to just patting or rubbing or whatever your sister used/uses to usually settle him). It sounds like he is somewhat comforted with a firm hand or being held down so this may be the way forward. Can you get the current EASY times and post them? Exactly what happens, WU, A times, S times.
If it is a very difficult phase and he doesn't respond easily and if she cannot bring someone in to to PDs for her, and if she really cannot find any position by the cot then my advice would be to start with where exactly she *can* get comfortable for a period. It might be more appropriate to set up a mattress on the floor for the pair of them to lie together for DNs nap time, or to use an open sided travel cot to put him in so she can lay on the floor next to it and once he is asleep she can close the side and leave him to sleep safely. Whilst these things might not sound 100% BW the reality of life and bad backs is very very real and part of BW is to consider the needs of the entire family, including Mummies who can't do a full on PUPD! it may take some out of the box thinking
hope this helps - I can look at the EASY times if you post them - he might be ready for a longer A time and it could help reduce the resistance to naps.