How can I get my younger sister to realize that not every student will like her?

So my younger sister is currently an assistant teacher. She’s quite and shy but she does greet them and smile. She told me she’s been working there for about three months and she said some students don’t like her. I mean she said that there’s students that ignore her, don’t listen sometimes and when she greets them kindly, a few won’t even look at her. She said she thought all the students will like her. In the class she works in at least. The kids in her class talk to the other three teachers but not her. Some do but they don’t talk to her. She also said a student said hi to the three staff, but when the girl saw her, she frowned. Some students that used to say hi to her don’t even say hi to her anymore. She said it stung with some students looked dissatisfied when they saw her but happier when they saw other teachers. I put my heart and soul to these kids and it hurts her that students prefer other teachers over her. She said a little girl was like, “No! No.”
So this is what I told my sister, “Well you just learned something. Not every student will like you or be fond of you. You gotta let go of that ego. Hopefully the experience will humble you more. Yea students should respect you and say good morning when you great them but don’t expect every student to like you. It doesn’t work that way.”

She got choked up and said, “Yes but one of my former teachers told me oh the students will love you. I thought she meant all of them. One students gave me a disappointed cold hi when I greeted him but when another teacher did he was smiling.”
Then I told her that her former teacher probably said some students. She should’ve told her that not every kid would like her and be prepared.. don’t go near a kid when they’re going though a hard time.
she still won’t accept it, so what do I do? She’s 20, and is autistic.

How can I get my younger sister to realize that not every student will like her?
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