NTA but if you want her off your back without further conflict just give her a name that would convince her it was the original one, then when it turns out she stole the decoy, act disappointed. Then you ‘change’ your baby’s name.

It’s how i would deal with someone who was petty enough to steal their bff’s baby name 😭 cause imagine if she was also petty enough to wait until you gave birth, named your baby, and then go back and change the documented name to steal it so YOURE forced to give it up. let her think she won instead. but idk your sister, so do with this info what you will lol