My boyfriends daughter always pouts and interrupts constantly how do i correct her w/o making her worse?

I moved in with my boyfriend to help him provide a nice place for his kids. His ex took everything. His son is very well behaved and sweet. His daughter is allways pouting, even when we are doing something for her (always) and interrupting anyone talking by being loud or blurting out answers. She is always trying to second guess me and will not quit trying to get attention constantly by being loud and being mean to her brother, even when she knows she will get in trouble. I don't know how to try to correct her behavior without upsetting her. ( the only time i said something to her she started crying and call her mother from her cell phone). So when she acts up, and pouts and s in on me talking i just stand there and have no idea what to say. Like im mute, and cant talk, or all hell will break loose. So i pretty much have to stay quiet all the time or i will get in on or if i say anything she will call her mother...help, i am building up horrible resentment