Relationship: Complicated!!

I need some advice...me and my partner have been together for a few years, I knew he had children already (now 14 + 12) That wasn’t a problem. As time went on it seemed the older one was becoming difficult...especially when I fell pregnant the 1st time, she screamed and stated she wanted to slit my throat!! And told her dad she hates him. Unfortunately that pregnancy ended badly and I lost our twin boys. Neither of us got counselling, he doesn’t even like to talk about it with me. We tried again some months later and now have a little girl. Things changed when she was born, he couldn’t deal with having a newborn around, didn’t think anything would change. Since then we’ve moved out, things between me and him are fine however his older child is constantly making things difficult for us. She will purposely ignore me when I speak to her, she will be clingy with him when I’m around, she won’t let him spend any time with our daughter. It’s got to a point where I’m going round to see him when his kids are at their mums house. The thing is I’m the one getting the blame because he doesn’t want to think bad of his daughter...I can understand that but at what point do I say enough is enough?! Sorry this is long but thanks for reading :)