AITA for wanting a family heirloom that was accidentally promised to both me and my sister?

My grandmother is about to move into assisted living. Because of this, she is getting rid of most of her stuff from her current home. My sister (23 F) and I (25 FTM) get first choice on many of the things my grandmother is getting rid of. My grandmother owns a piano that used to be her mother’s. It is something my family has cherished for a long time. I am a musician and when I started going to school for a music degree my grandmother promised me that she would give me the piano either when she passed or when she moved into assisted living. Last week I found out that she had also promised the piano to my sister a while back. This wasn’t malicious or intentional on my grandmother’s part. Her memory is no longer the best and she did not realize she had promised it to both of us. My sister and I are now trying to figure out who gets the piano. My sister argues that she should get the piano because she has a long term partner and they want their future kids to learn to play the piano on my grandmother’s piano. I argue that I should get the piano because I’m the only one who is actually a musician so I would already be using it long before my sister and her partner have children. My mother says I shouldn’t want the piano because I am a vegan and the piano has ivory therefore making me a “bad vegan” if I keep the piano. My response to that would be I didn’t buy this extremely old piano and it is a family heirloom that means a lot to me. We haven’t brought this dilemma up to my grandmother because she has declared that if we cannot decide who gets something of hers she will sell it without question and no one in my family wants this piano to be sold. It feels like my family is on my sister’s side and they think I am being selfish for wanting this piano even though I am realistically the person who would use it the most. So, AITA for wanting this precious family heirloom?