I like the various suggestions for alternatives. These could be great solutions.
However, I think most people aren't really listening to the OP, who has said twice that at least part of her would like to call her SM mum and that her worry is betraying her mothers memory, rather than not feeling close to SM or not wanting to.
OP, if you don't like the feeling of calling her mum, then you absolutely shouldn't and don't have to. But if you actually would like to have that, and she is a long term presence in your life and does act towards you in a motherly way, then you should feel absolutely free to.
It sounds like you are worried your late mum might be upset or feel betrayed. We can never know this, but as a mum I can say that if I died, my most fervent hope would be that another caring, loving woman would act as my proxy- so my daughter had a motherly figure in her life.... if my daughter felt close enough to someone to call them mum I would be profoundly glad. I would be dancing with joy and sending all my love and blessings into that relationship. Having a permanent gaping hole in my daughter's life is not how I would like to be honoured. Her being happy would be my only wish.
Really it comes down to what you want - but I wouldn't let guilt stop you, if that is your blocker.