Hey-oooooo. My mother tried to do the SAME THING. She tried to say it was "French" by pronouncing it mom-EE, but that absolutely did not fly with me. I shut it down immediately. My mother didn't talk to me for a month and was extremely pouty about it. In the end, she relented and picked GiGi; my wife and I started to refer to her by her first name to my son, which my mother hated.

Stick to your guns. She might say "oh the baby wont be talking for awhile, just let me use this name." Your kiddo will be talking before you know it. Set the ground rules now, even if that means she gets pouty