I became a stepmother last August, when my fiance moved in with me. We had my 4 year old stepson 50% of the time.
In March, it became official, when I got married.
A week after that, we got full custody. His bio-mom doesn't even have every-other-weekend. She gets him 4 hours per week. Supervised.
I am now a mother. Step or not, I am a mom. I feed him, I bathe him, I read to him, I kiss him good night, I dress him.