Are those soft bones osteoporotic? Luckily there are some good medications now (Tymlos and Forteo) that build strong bone. Surgery for scoliosis involves rods and screws. If your sister's bones are too soft they will be unable to hold the hardware. Both of the mentioned osteoporosis drugs help integrate the hardware in addition to making the bone stronger. I'd get an endocrinologist involved before getting second opinions from orthopedists.
There are newer rods that can move which are better than the older hardware.
Painful death conversations are manipulative, still surgery might be a good choice.
There is something called Clear Scoliosis that you might want to check into. It is a system of bracing and exercise.
Best luck