AITA for being angry my sister called my baby a pet name?

So my (27M) son is called Nathanial, my dads middle name and my mum's dad's name, I know it isn't the most popular name but my wife and I love it.<br><br>My sister (26F) has a daughter who's 3 months older than Nathan. She has always wanted her son's name to be Atticus and on the off chance she had a boy, she would've named him Atticus. But her child is a girl, and has been named Matilda. She often jokes that Matilda is really an Atticus on the inside and even calls Matilda "Attie".<br><br>My problem is that she now also calls Nathan "Natie" which is basically the same nickname. I told her I didn't like it and would prefer she called him Nathan, his actual name, and she said okay but then she forgot.<br><br>Nathan has recently started babbling and saying the odd word, but mostly he just tries to mimic what people are saying. When my sister called him Natie, he copied the word and said it back. I reiterated to her that I did not want her calling him that, especially since he's now saying it and she told me to get over myself and wear ear plugs. She said that she was just having a little joke and didn't see what the big deal was.<br><br>It is a big deal to me because she is repeatedly going against my wishes. I am extremely close to my sister and we are more like best friends than siblings. She's the only person I fully trust with Nathan. But I do feel like this is disrespectful.