Getting your toddler prepared for new baby?

Im 26 weeks and i have a 3 year old daughter. My husband & i have been keeping her involved with this baby. She calls him her baby tummy & talks to my stomach & even rubs my stomach. I thought she was going to do great with her new little brother but my niece has come to live with us & she is 1 & my daughter has been HORRIBLE towards her. Snatching things away from her, getting jealous if i play with her, pushing her or scratching her face if she gets mad at her & screams at her all of the time. Because of how she has been with her im worried its going to get worse against my son cuz of how she is with her! Is there anyone thay can give me any advice on how to help with her towards my niece & my son? Thanks in advance!!