Im not sure how i feel...

So my SO has two kids already, a girl and A boy. He doesn't have custody of them but he has already said that I'm step mom. I love him more than anything but I feel weird being A step mother. I know how I felt towards my step mom and don't want his kids to feel that way. I don't want them to see me as a step mother but more of a friend kind of person. Is that wrong?