Very blended

Need some advice! I'm in the process of a step parent adoption of adopting my spouse's child from a previous marriage. My spouse has already adopted my child from a previous marriage because the other parent had no contact. We have also had a child together. What's unique about my step parent adoption is that my step child's other parent is in a relationship with someone who abused my step child and for the past few years they haven't had any meaningful contact and it's really just a big mess and this other parent really puts on a show saying how much they love the child but of actions mean more than words then those words basically amount to nothing. My step child really clings to hope that the other parent will come through for them and I think it's because of this hope they have such an easy time manipulating my step kid. And my step kid believes every single lie they are told it's kind of heartbreaking really. The other parent is a bit of a narcissist. But what I'm having trouble deciding is weather or not it would be best to continue contact or just make a clean break and hope and wish my step kid will understand our logic one day. I just want what's best is it best to continue this toxicity in my step kids live because what I'm reading birth parent contact is more beneficial.