Yes, they will consider her physically unable to perform her job, which could be a safety concern for both employer and employee. I would instead concentrate on getting her social security disability benefits. It's a long drawn out process of about six months but if the medical condition precludes her from even the simplest of jobs, she should be considered eligible. Go to www.ssa.gov and click of disability benefits

If your sister is unable to perform her duties, for whatever reason, an employer is not obligated in any way to keep her on the staff. If you live in a state that employs "at will" it means the employer is able to fire with little and sometimes no reason at all. From the employer's viewpoint, if an employee cannot do the job they were hired to do, they need to find someone who can.

Perhaps due to her condition, your sister may be able to speak to her doctors about at least a partial disability option. If your sister can't work, she may be qualified for such a program. If she can work, she has to take into account what jobs she would be able to do without provoking her medical condition. Continuing to work at her current job may not be good for her health. Perhaps she remain employed if she can do a different job within the same company, one that doesn't require her to do the things she does now.

Your poor sister, I took have a bad memory, and when I was young I had a few brain bleeds, my surgeon operated to stop the bleeds, but I also have a cyst on my brain, they may have to let her go due to ill health, but first they have to go through preceedures, like paying for private medicals, so the firm is covered in case your sister has a fall or anything, when I was at work I was unwell for quite a while, they kept my job open for me for quite a while to see if I was getting better, but I didn't so they let me go, but they gave me 2 months pay, and they said if I did get better I could re-apply for my job back.