Please help with my 5-yr-old before I lose my mind

[corinne76]

DD just made 5. I feel like I'm constantly yelling and scolding her. She's always taking things away from her brother, yelling at him. And when I ask her not to do something, its like she doesn't hear me cause she does it again right after. I'm going to lose it soon because I can't get through to her. I know she doesn't always mean to be defiant, like her brain does not process a command in enough time or something, but it irritates the heck out of me and I'm constantly exasperated.
Any advice to deal with this phase, or ways to help correct her like get her to actually listen would be fantastic