There's really no way to determine in a forum such as this why your daughter is acting in such an aggressive manner. However, the behavior bears some intervention on two levels. On the behavior management front, closer supervision of her will be required. She needs to be prevented from being in situations in which she has the opportunity to hurt her sister. At the least sign of aggressive behavior, she should be placed in time out, preferably in an adult-size chair. You may need some guidance in how to implement a plan, and you can obtain such guidance, as well as an assessment of your daughter, via an evaluation by a pediatric mental health clinician. The clinician may meet with your daughter face-to-face, and will also want to talk with you about the situation. Often, but not always, when such youn children act in such a persistently angry way, there is something amiss in the relationship between the child and parent. This might warrant some consideration on your part.