Thank you for your email. I hear your concern that your daughter may be a bully.
It is important to understand what is going on with your daughter and while I would not confront her about the bullying especially if she denies it, I would urge you to give her a safe space without criticism and judgement to express her thoughts and feelings about friends, siblings and anything else that may be bothering her. It is important to establish if she is being bullied in any way or if there is anything else that is bothering her.It is also important to ascertain if your daughter is role modelling an adult's behaviour in her environment or emulating a "cool child" at school. At this stage give your daughter the benefit of the doubt and your support while gaining as much information as you can without confrontation.
In addition, it is important to teach her empathy; being able to put yourself in someone else's shoes and understand what they are feeling and how your behaviour impacts them. You can do this while watching TV, talking about your friends, reminiscing about your childhood, when you and her witness two children fighting. Empathic people find it very difficult to inflict pain on someone else as they understand how the other person is feeling.
Your daughter is also at an age when it is appropriate to teach her conflict resolution talking about techniques you and your partner have seen work effectively in both your personal and work environment. Also emphasise how important it is to stand up for someone being bullied. You and her father are the best people to advise her at this point and you may want to consider reading many of the available books with her. See Kalahari.net. In addition, it may be prudent to chat to the school about what anti bullying programmes they have in place which are usually run by the guidance counsellor and or school psychologist.
Let me know how your chats with your daughter go and especially the information she gives you, and I can advise you further.