Because she knows that if she holds her breathe long enough she can get either of you to do what she supposed to do. My oldest was like this and still is in many ways (she's 12). I outright refuse to have my child dominate my life - a lot of my friends think that i'm too harsh but I know that I didn't have the option of telling my mother what I was and wasn't going to do.
My oldest has had many punishments and she still hasn't caught on. The most recent was last summer we were all cleaning out my van because it wasn't all her mess & she shouldn't have to clean it all. So she fussed so much that we had plans to go down to the shore overnite so I made arrangements for her to stay home with my mother-in-law. Maybe it was a bit extreme, but if I don't stick to what I'm trying to show her then she'll continue doing this forever. One of the hardest things for me is to stick to the punishment, but after you do it it knocks them down a few pegs and makes them realize it. My daughter has had sleepovers cancelled at the last minute for her attitude and I know that maybe I should ease up a bit, but it just gets so frustrating. Good luck!