A different perspective from myself might also help.
I am technically a SM and have been a part of DH's child's life from a young age.
So, in reference to the posts above I have been involved as what might be described as a parental figure.
However I don't use or like the term SM.
I am "DRM" to DH's child, also referred to as "Dad's wife".
I'm a mother to my own child and frankly wouldn't like (if such circumstances were to occur) for anyone to describe themselves as a mother to my child - whatever pre-fix they used.
DH's child has a Mum. She'll only ever have one Mum and this imho is how it should be.
I've a very good relationship with DH's child (now in her 20's) so please don't misconstrue this as part of any "distancing" on my part. Rather I think it's helped us to be closer because there's never been any ambiguity about the nature of our relationship. I'd be there for them in a heartbeat if they needed me, but I am not and never will be a "mother" in any sense. I'm a very supportive and loving adult member of her family however.
It's not somethings that's ever really been discussed, rather it's how I've described myself and everyone else followed.
So as I said a different perspective and I think that as an adult you are quite within your rights to define the relationships in your life.
Ultimately it's the quality of the relationship that really matters, not how you choose to name it and your DF can "dress it up" as much as he likes but she's not, never been nor ever will be a "mother" in any sense to you. That's not being nasty, it's just a fact.