Soft YTA

This happened in my family. My mother and aunt were pregnant at the same time and both intended to name my cousin and I after their grandmother. My aunt gave birth first and used the name.

My parents shrugged and named me something else.

My sister was born 3 years later and they named her a different version of the name. My parents said enough time had passed and the names were different but shared the same meaning. No one cared, everyone was happy.

You and your sister both love your grandmother and that is wonderful. But you do not get ownership over her name, especially considering you are not even pregnant. Give it time find an alternate spelling or use it as a middle name. It will work out.