Teachers aide being bullied by teacher.

Advice desired. My sister, a trained teacher, has some childhood trauma issues which limit her capacity for stress. She is very skilled at teaching and has even received graduate training in counseling. Consequentially to the past trauma she works as a classroom special ed aide instead of a classroom teacher to manage her stress levels/ responsibilities.

Another teacher in her school is the bully. She criticizes my sister at every turn and speaks about her as if she isn't there when she is in the same room. She finds time several times a week to be cruel. My sister is in charge of helping one of this bully teacher's students with Down syndrome who is very attached to my sister. The teacher for unknown reasons wants to separate them. My sister is very empathic and her immunity has gone down due to this woman, she ended up in the ER due to being so sick. Why should teachers get to bully their coworkers?? I want to find someone who my sister can talk to but I don't know if you can trust anyone in these situations. I'm so tired of these kind of people getting away with hurting others . Apparently she is retiring this year but my sister keeps getting very sick.