Why does it matter when your W told your D what she did? When your daughter confides in you, please just listen to her and reassure her. Make it about HER, not what YOU want to know.
While it's true that you are not yet divorced, it is true that your W might remarry and then your daughters will then have a step-dad. I would have told her that if and when mom remarries, then yes, that man would be your step-dad. Then reassure her that you'll always be her dad, lover her, help her and be in her life.
Focus on your children and what they need from you to get through this difficult time. Wasting too much time on anger is not helpful to you or them. Stop giving your power away to your W.
