M.P.
M.,
Have you tried rewarding her when she is behaving the way you want her to behave? Many parents only give attention to their children when they misbehave. This can escalate the wrong types of behavior. It sometimes takes several tries to change a behavior. But consistency is the key.
Though she is only one she already knows how to get your attention. You may have to sit her down alone, without her brother, and give her some direct attention. i.e. reading a story. Tell her how well she is behaving . (Catch her being good and reward her with a hug or a pat on the head.)
Also, you should know that, children don't always behave at home the same as they do elsewhere. It is not unusual for a child to be great at home and awful at school or daycare, and vice versa.
If you want a behave to change you have to change the focus. So focus on the good behavior and reward it. Be consistant and it will change.
Good luck,
Mamapri