I'm going to tell my sister she shouldn't yell at my niece so much
Do you think she will stop yelling at your niece?
I hope but I doubt it.
It is good that you want to protect your niece. Your sister shouldn't yell at her, it is not a good way to communicate and it won't fix anything.
It's not her fault as it's due to her social circle. She hangs out with a lot of jerks.
Regardless of who her friends are kids should not be yelled at. Can you tell me what you plan to say to your sister before the confrontation?