Step daughter

My husband was married before me and had a daughter. Before I had my daughter me and my step daughter had a great relationship. Once I had my kid, our relationship went sour. It's basically  non existent. I have a lot of problems with her mother that caused me to back off and only focus on my daughter. I barely acknowledge her when she's around. She's only 7 and I know it's not her fault that her mom is a horrible person. It just causes me anger and hate when dealing with the mother that is hard not to be angry at my step daughter. Being a mom is hard but being a step mother is 10x harder. I have to treat her the same as I treat my own daughter but have no say in any decision. And my husband has little to no back bome when it comes to the mother. He gets walked on and the daughter is the one who suffers. Recently I have been working on the strained relationships in my life and  this is one relationship that needs help and I need advice. Please help