Why does my almost 3 year old niece treat me so terribly and back chat to me

[Nicole]

with such disrespect? My niece will be 3 in April. She is a very spoilt little child and is given anything she wants when she wants. My sister will always give in to her, her father is not consistent in her life and my sister feels like it' her fault or something. My sister has always told yelled at me when i have said 'no' to my niece when she was just a baby ( i half raised that child to the 3 years she will be in few months time) I should be seen as a parent to the little child i used to be until i started working alot and only seeing her on weekends (we used to live next door to my sister, to help her out with the baby when she was a single parent). But today my niece shows no respect to me what so ever. I dislike spending time with my sister because my niece is so damn nasty to me. If we are at my house and i give her an instruction and i speak to her in a way which my sister thinks is wrong, she will shout 'don't speak to her like that'. My sister constantly says it and has been since her child was very young. Now my niece screams in my face constantly and constantly ignores me and constantly disrespects me in public. She gets away with murder that child. I hate it, i can't even have a conversation with my sister without my niece shouting at me because she thinks i am say 'bad' things about her mother. I can't even call out to my sister from another room without a 3 year olds rebuttal she will shout at me and say 'no Nic!' and then look over at her mother for approval 9/10 times my sister will ask, why are you fighting with a 3 year old'? or she would say 'just don't talk to her like that and she wont to you" or something absurd like that. My niece is always smerking at me because she knows she has complete power. This child is out of control in my opinion and i don't even want to be around her because she really just doesn't appreciate me or even respect me. This child is very smart ( a high IQ) she has been tested to have the mind of a 4 year old (that was when she was 2) I have been having conversations with her since she was 16months old. I loved that little baby now i still love her but i dread the time i have to spend with her. Mothers out there or someone what is happening???? What should i do? Am i over reacting? I am 22 years old and i live with my partner we have always had our own house and my niece has always visited us and stayed the night with us. My sister is 28 and she has had a new partner for the past 6months but my niece seems to love him more than she loves me now, unlike before. I played a major role in that child's infant life, i was a parent figure for such a long time me and my partner.