constant fighting with each other, not listening to anyone, hitting each other, blaming her little sister for the thing she didn't do, deliberately leaving the door unlocked to get me in trouble and her little sister
Answers (1)
First off, use a firm voice. One that has a sense of authority.
Secondly pick the one who is the source of the problem, and call them by their full name.
Finally, give an open ended consequence. Something that makes them think up the worst case scenario.
So for example:
"<First name> <Middle name> <Last Name>, you and your sister better stop fighting or you're not going to like what happens next!"
If the other decides to take advantage of the situation pass it on to her by saying something along the lines of "That goes for you too". Seeing her sisters reaction and then having the same, yet shorter threat will have the same effect on her.
That how to stop the fighting. Keeping them from fighting requires a combo of positive and negative reinforcement.
Suggest a reward to them to get them to be good.
Set them in time outs if they're bad.
This is all with the assumption that they're young nieces. You didn't really give a lot of info.