I love my son but I don't always like his behavior.
He is almost 15. It is a horrible age for him and he is not wearing it well.
Oh, and the facial expressions are sometimes so grotesque to be amusing to me - especially when he gets the "I am so bored, disgusted at the world, you are all just a speck on the bottom of my shoe" look.
I make it clear to him, that I love him, but it is the *behavior* that I don't like - I don't want him to get confused and think that I don't like him, hence, in his convoluted teen mind, don't love him.
My family, like yours, all correct each other. Begin a dialogue with your niece about her behavior. Call her out on it when she is being rude and disrespectful. Calmly and gently - my Granny always said "you get more flies with honey then with vinegar". LOL
Let her know that your love her, but do not like her behavior, and that you know she has it in her to behave better. After all, she knows the "rules" she is just choosing to ignore them. Tell her that how she behaves and expresses herself with her friends is one thing, but how she behaves and expresses herself amongst family, especially elders, is another.
She does sound like a typical almost teen - these are not pretty years for them. Hormones, burgeoning sense of self, developing maturity, peer pressure, the list goes on.
Other people assure me that I will get my sweet son back one day - probably after college, and many gray hairs later.
It is wonderful that she has such a caring Aunt. And while she does not seem to appreciate it now, truly, inside she knows how fortunate she is.
Good Luck
God Bless