Although, employers can hire and fire "at will", what kind of progressive disciplinary action was taken against your sister? What type of performance rating did she receive? If your sister was an outstanding performer and the revelation of her disability to her employer resulted in her being fired, it's called disability discrimination. NO EMPLOYER can terminate an employee because the have a disability. They must have a "legitimate business" reason. The reason given to your sister by her former employer that "she could not handle added responsibilites" is not a legitimate business reason if it is not documented in her official peformance appraisal. Although, employers don't have to give them, if they have a policy of giving annual reviews and they don't follow it, they could be held liable. Progressive warning is needed to give an employee an opportunity to improve his/her performance. Your sister should get copies of all her past performance reviews and any documentation she has regarding her assignments. She might have a case. Don't listen to EX HR-GUY. I've read the ADA myself and I know about performance reviews first-hand. I've contested them negative ones and had them overturned and subsequently kept my job.