How do you decide what LO will call its aunts & uncles (and what your nieces and nephews call you)?

I grew up just calling my aunts and uncles by their first names. DH grew up calling his Aunt Firstname and Uncle Firstname. When DH's sister had a baby, we became Aunt Lady Baltimore and Uncle Lord Baltimore, because that is what SiL taught my niece to say. I prefer just to be called by my first name, but didn't feel too strongly about it, so whatever.

Fast forward several years, and now LO is learning to identify her relatives. I just refer to her aunt and uncles by their first names, but SiL is teaching LO to call her "Auntie." This is probably not a fight I am going to choose to pick, but there is a part of me that feels like "hey, when do I get a say in what people are being called?"

So, who do you think should get to decide? The person being addressed, or the parent?