Standing all day at work

What exactly are you asking?

Think of it as motivation to get promoted to a job where she doesn't have to stand all day.

Sure why not put her out the back shoveling coal, great motivation for anyone

If your sister doesn’t want to stand all day at work she probably shouldn’t start a job that requires standing all day at work ?

Page 27 of this HSA document states that

Working in a standing position for a long period can cause sore feet, swelling of the legs, varicose veins, muscular fatigue, lower back pain and stiffness in the neck and shoulders. Seating is required where most of the job can be carried out while seated or where there may be opportunities for workers for sitting down between tasks without detriment to their work.

From what you say, your sister could carry out her job equally well sitting, on an appropriate high seat, as she can standing. IMO she should approach her union and let them take it up on her behalf. The union can raise the matter with management, and the HSA if necessary, without referring to her or drawing attention to her as a potential troublemaker.

actually standing desks are also becoming more common in offices aswell. Also teachers/lecturers etc are on their feet most of the day aswell. I work in the public sector and I have colleagues are on their feet most of the day. No issues at all.

I have this exact issue in my workplace where there's little movement and suffer from pain in both my lower back and knees. Thankfully I wont be there much longer

This position is based at a standing height table in the public area of a bank. This is nit a place where you could be seated and look professional. The person has plenty of opportunity to make small movements during the day, while assisting customers to step over to the machines etc.