The Girlfriend vs The Baby Mama

My boyfriend and I have been TTC for a few months now. He has a 3 year old by a woman who should be called a lot of things but a mother I would tread lightly. She doesn't have custody of any of her children and is trying to make it hard for my bf to see his daughter. 
I've endured the Facebook threats and rants and I'm like heffa u have beautiful girls and can't even see them when you want. Long story short I sometimes worry my boyfriend will feel like I'm trying to replace his child with a new one but I'm not. 
Just venting but what do you ladies think?