Dear Laia,
My baby sister (strictly speaking only half sister) went through a similar situation when her parents divorced when she was five years old. It’s understandable that your step-daughter is difficult during the first hours of your contact. She is confused and probably doesn’t want to lose either of her parents. She may be afraid that by loving you and your partner, she may lose her mother. That’s why I think you’re doing exactly the right thing. You talk about her mother, not in a negative but in a positive way. You let her express her feelings. Give her the feeling that she is loved unconditionally. Give her the feeling that she doesn’t have to decide.

I don’t know if there is any possibility to ask the mother for a diplomatic and peaceful conversation on how to co-parent, communicating to her that you don’t intend any harm. Maybe even with the possibility of a mediator? But probably you’ve tried that already?
You cannot control the mother’s behaviour. You can only do your part, but you can’t change how other people behave. And it’s sad because you can see how much pain it’s causing, but you can only control your own behaviour. And by giving your step-daughter a place of love and positive communication around all parents, you are making a huge difference.

My sister is now a teenager and though she’s struggling in some aspects, she has become a very confident, strong and resilient girl, despite all the negativity she experienced. And I’m very certain that that’s because there were people in her life who gave her unconditional love and understanding.

Stay strong and positive and trust that you’re doing the best you can in your situation.

Much love