I don't like ex's wife being called my children's step-mother. She isn't their 'mother', step or otherwise, she is their dad's wife. If she had brought them up, either because their father got custody or I was dead, then she should be called their step-mother, but not if all she did was marry their dad. Similarly, I don't expect my future husband to be called their step-father - 'mum's husband' is sufficient, hardly a mouthful.