What your niece needs, is a "relationship" with her Mom.
The girl, obviously, is lacking in what she needs... emotionally.
Meaning, she needs a Mom... to confide in and talk to and chat with and share her hopes and fears and dreams with.
When a child lacks this, in their parent... they act out.
Because, there is no connection... with the parent.
No physical or emotional, closeness.
And thus, they lack an "anchor."
If a parent only treats all things, with only a punishment or reward for "behavior"... then what is MISSING is... the parent does not have a heart to heart "relationship" with their child. Because, all interactions with their child only is contingent on rewards and/or punishments. But it does NOT, nurture or develop any sort of relationship with the child. And the child then does not feel any camaraderie with the parent.
So then they get "lonely" or feel, not close to their parent. But any child, no matter how old or young... needs a Mom. To be close to... despite any happy or grumpy feelings they might have. A child, needs a Mom to chat with, about anything. Without fear of punishment or rewards, hanging over their heads.
Many times, scolding or punishments or rewards, becomes the basis of any parent/child interactions.
But this just overlooks, a child's actual needs or lack. Emotionally.
And it then creates a dynamic where the parent doesn't even "know" their child.
Your sister... being her daughter is 11, needs to nurture her daughter. Not think of her only in terms of punishments or rewards.
She needs to, see her daughter per her development and age, and try to KNOW her daughter... despite difficulties. So that they can become close. As a Mom to a daughter. Not as "friends."
A Mom, has to be there for their daughter. Even when the daughter is having difficulty. Not just punishing.
With my daughter, I KNOW... if she is just being difficult or if there is something else, churning underneath the surface.
And I am there... for her.
We are close.