My step-daughter is 24 now. I don't know the acronym for step-daughter - DSD?
Anyway - I was with DSD's Dad for about 5 years before me and her Dad, ex-DH, had DS2.
Therefore, I have a DS1 of my own, he had DSD and we had DS2 together. So, I had been with ex-DH about 5 years before I even met DSD, as he had not had a relationship with her since splitting with her mum when she was 2. She came to live with us when she was 12 after her mum left her with her step-dad. My own DS1 was 9 at the time and I was 3 weeks off giving birth to DS2.
Ex-DH left when DS2 was nearly 2 and left her with me. I felt really at sea as I hadn't even known her till she was 12 and spoke to her school saying that we had had upheaval and to please contact me if she was being naughty cos she was probably just feeling scared. I was really shocked when they asked if I was her mum and was I "Mrs So and So" I said, "No, I'm her stepmum - my name is Mrs This and That".
They said she had always referred to me as her Mum and told them how kind I was. Until that point I had kind of been "baby-sitting" and half thinking - "this isn't even my kid".