Working with a therapist is a great idea. He or she will help your stepdaughter deal with the changes she has experienced, as well as help you with suggestions for some behavior-management strategies. You might also want to talk with the school counselor at your stepdaughter's school; the counselor may be able to give her some individual time or include her in a small group on having a new baby in the family.
Think about your stepdaughter's behavior as her attempt to get attention. For some children, it doesn't matter if the attention is positive or negative as long as they get it. Try giving her attention when her behavior is appropriate. Include her in tasks with the baby (she can fetch diapers or help with bath time) so that she doesn't feel left out.
Look for children's books that deal with having a new baby in the family and read them with your stepdaughter. Make sure that she gets some time alone with her dad and with you when she has your full attention. It may take some time to address these problems, but hang in there and don't give up on her.