Would you let your step child call you mommy?

My step daughter is 2 and half. I've been around since she was 4 months old. She knows who her mother is and she knows who I am. She nearly always calls me by my name, however recently she's started to call me mama. I started to correct her but then I realised she does it mostly as a comfort thing. (When cuddling me) I've stopped correcting her now. I'm having her little brother in 9 weeks, and I will definitely be called mommy by her father when the baby is born (just as I call him daddy now) is it mean to tell her she can't call me mommy too? I don't want to take away from her mother at all but I don't want her to feel left out either? What would you do? 

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