J.D.
M.,
The first thing to do would be to take her to see her pediatrician. It occasionally happens that a sudden change in temperament can be linked to something medical, like adeveloped allergy to something in her diet or environment, and it's important to rule that out.
Second, it is apparent that something has changed for her that is causing some kind of stress or anger. Talk to her teacher to see how she is doing in school, both with her work and her behavior. Is she getting along well with the other children? Is she acting out in class? Sometimes a kid who is having a social problem at school acts out her negative feelings at home, where she feels safer, because she's surrounded by people she knows love her, even if that's not what she's saying.
If neither of those lead to some insight as to what is going on, then talk to her doctor about a referral to a psychologist who specializes in children. A lot of these doctors work through play therapy, and it can be remarkably successful. It may be a maturity thing. She's only six, and she may for the first time be truly understanding your family dynamic, particularly with having s "step". A lot of the kids she goes to school with have step-parents I bet, and if they are displaying dislike or contempt for that person in their lives, your daughter may just think that's the way she's supposed to feel about him. A psycholigist will be qualified to work with her on getting an explanation of what is causing anger or unhappiness, and be able to tell you how to work through it, and how to discipline though it, as well, because you still have to. No matter what her problem is, it's NOT okay to take it out on her little brother.
Best of luck M., to you and your little girl.
Jess