Problems with his first child and our new baby

SORRY THIS IS LONG BUT PLEASE READ I NEED HELP!

My fiancé and I have been together for 4 years now and just found out we are pregnant with our first child together two days after Christmas!! My fiancé also has an eight year old daughter from a previous relationship. We all have a very close relationship, and my fiancé’s ex and her new boyfriend even spent the night Christmas eve at our house so that we could all be together, and so that my fiancé could FINALLY experience the joy of waking up with his child on Christmas morning. My fiancé’s ex accidentally found out that I was pregnant when she walked into our house unannounced one day and spotted my, “What To Expect When You’re Expecting” book on my end table. She was over the moon happy for us both, as was his daughter as we all hugged and celebrated the new addition to our family together. However, let me back up a minute. My fiancé’s ex was married for 4 years, but is now in the process of divorcing, and has already moved herself and my fiancé’s daughter into her new boyfriend’s house almost an hour away! She has already enrolled her into a new school, which will be the third school she’s been to in a year and a half. Because of all these changes to her life we did not want to tell her right away that we are pregnant as to not overwhelm her with too much change in her tiny life, but since that was ruined we really didn’t have a choice. Now my fiancé’s daughter is becoming more and more angry. Last week she pushed her mom’s new boyfriend down the stairs in a fit of anger and his back was seriously injured. When she was with us last weekend she argued with nearly everything that came out of my mouth and it just seems like she’s craving attention so badly that it doesn’t matter what kind of attention she gets. We’ve tried almost everything to make her feel special. We try not to talk about the new baby and if we do we tell her how special it is to be a big sister and that her father and I, and her new sibling will really need her help. We spend quality time with her, we take her out to places to do special things with only her, but nothing seems to work. I have suggested outpatient therapy, but at the end of the day that’s all we can do is suggest because his ex has primary physical custody of the child. Her anger is getting so bad that I’m now fearful I might be the next one getting pushed down a flight of steps. Her argumentative nature with me is becoming extremely stressful which is not good for me or the baby. At this point I’m ready to tell my fiancé that maybe it’s best if his daughter just stays at his mom’s when it’s his weekends with her and he can go back and forth because his parents only live two blocks away. So my question is...am I wrong for feeling this way? If anyone has any suggestions on how to move forward it would be greatly appreciated!!!!