Would you step in & get her to listen?

My niece is 2 1/2yrs old. She has a total disregard for what anyone tells her! She does not listen when her mom or dad tells her no. She does not listen when her Grandma & Granddad tell her no. She simply smiles and continues on w/whatever she is doing. And the screaming & crying! If she does get taken away from whatever she is being told to stop doing..OMG!! The world is ending! She screams & cries & has made herself sick! And, of course, all the above give right into her! But there is someone she listens to...Me! When I tell her NO she stops & there is no fuss or muss about it! No crying! No screaming! She stops & I show her something else to do. Now, there have been times when I have been around & her mom is telling her NO not to do something. And she simply continues on. Mom repeats over and over and over NO..but she does not listen! My husband has heard this and said to me to say something to my niece (knowing that she will listen). But this is my ?...I feel "bad" jumping in and telling my niece NO when it is not my place! I am not her mother! And, in my opinion, it is her mom & dad's place to get her to listen! So what would you do? Would you jump in & make her stop? Or would you simply sit there and let her continue on w/what she is doing? Or would U wait & c if she listens & if she does not (after a period of time) then would you jump in??