Identify what she values most, and reward her for good behavior on a daily basis for how she treats her siblings.

If it's a game, toy, friend, TV, whatever it is. Each day that she treats her sibling kind, she is allowed to play or use the favorite item. If she is mean, she loses access to that item for a day until she can earn it back. No negotiating, just a simple rewarding good behavior with something she already loves today, and punishing bad behavior by removing something she loves today. It has worked for us.