I agree 100% with Julia M.
Furthermore, consider this:
Put the situation on its head. Pretend this isn't your niece, and isn't a member of the family. Pretend that the person your sister is screaming at is her best friend, someone she loves deeply & who loves her. Your sister starts screaming at her over something her friend has done. Her friend, as an adult, would probably do one of two things: Either stand there and take it, in an emotional dead pan until your sister stopped and then either present her case calmly or tell her she'll talk to her later and walk out before she lost her own temper. OR the friend attacks back and then probably storms out. What would YOU do if a friend started screaming at you? Especially if you didn't feel you warranted it? You'd probably either think she was going through some major sh**, or had become unhinged. Possibly both. Your niece doesn't have the option of walking out. At least, not yet that she's realized.
By going deadpan your niece is actually taking the high road, AND showing your sister a great deal more respect then she probably deserves. If an adult is screaming at ME it had BETTER be because I'm in imminent danger of dieing or killing someone, or because I'm really far away and they're trying to get my attention.
Your niece isn't being taught how to be a confidant, self assured, problem solving adult. If you're LUCKY she'll move far away as soon as she possibly can, blossom and come into her own strength, and decide to come home and continue a relationship with her family on adult terms.
Hopefully her grades will continue strong enough to be able to do this at college. OTW the military is another way to get out of town fast. Of course, if you're not lucky, she either won't want to start a new adult relationship with her family again, or she'll turn to drugs, or she'll get into an (or a series of) abusive relationships. Although, that will probably happen anyway unless SHE'S lucky AND smart. After all...Imagine a boyfriend who treats her the way her mum does. Not a pretty picture, but one she's being trained for.
I wish all of you the best in this situation, and I'm SURE you all love her. It's unfortunate she's being taught not to respect herself or trust others...& I reeeeaaally hope you may turn into the person she does trust. One psychological "trick" to help install that (if you decide you want to be her person she can trust...which means you have to choose her over your sister...and that may not be something you're willing or want to do) is to decide from here on out that your niece is an adult. If you THINK of her as an adult, and talk to her like an adult, you may start getting some very adult responses back...because she'll start modeling the way you treat her and start treating you the same way. You have to really believe it yourself though...otherwise your tone and body language will be off.
Good Luck, to all of you!!!