Any chance you could talk to your stepmom about your concerns, and what you are thinking? Maybe you could lay it out for her and give her a choice. You could tell her that you want her to be your daughter's grandma, but you don't want your daughter to be hurt, in case grandma doesn't act like your daughter is her real granddaughter, because kids pick up on that stuff even if they don't understand the reasons. So then she could choose whether she wants to be a real grandma to your baby, or if she would prefer to be Aunt X instead, and you will both acknowlege that there is a difference and accept the relationship as it is. First, however, make sure that you can accept whichever answer she gives. If there is one or the other answer that you won't be able to live with, then the other is probably your choice.

Rap
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“We should always pray for help, but we should always listen for inspiration and impression to proceed in ways different from those we may have thought of.”
– John H. Groberg