If she cannot deal with negative comments then she needs to not be doing teaching. She will have to deal with sickness, marriage breakups, death and everything else that goes with life.
What will happen when a parent questions her teaching or something that has happened in the school yard?

We would all love to live on a perfect world but that's not how the world is.

At 23 she needs to grow up and live in the 'real world'.
This! My first thought was what the **** is she doing teaching for if she can't handle anything negative? She is going to cop negative all the time - from students and from her co-workers. But I also agree with what nicnac said - maybe this is coming from your sister who wants to protect your niece from bad stuff? Unfortunately, she does need a dose of reality. If your Mum is really sick, then your niece has a right to know that because if your mother gets sicker etc, then at some point she is going to find out and it sounds like she is going to need time to prepare for that because it will hit her like a ton of bricks and she won't have the coping skills because she's never had to have them.