AITA for not telling my sister the name chosen for my unborn son because she used her BBFs baby name for her daughter?

My sister and I are both pregnant. This is her second child and my first. We’re both having boys. When my sister had her daughter three years ago her BFF was pregnant at the same time. My sister complained for 8 months that they didn’t have any idea what to name my niece and then all of a sudden she had a name right before she gave birth. After my niece was born and her name was announced, my sister and her BFF started fighting. The BFF said that was the name she’d chosen for her baby girl and my sister used the same first and middle name for my niece and she couldn’t believe she’d do that.

My sister said it’s first come first serve and she needed a name badly. That her BFF had time to find another name.

My sister is due before me, a few weeks before, and with that in mind I don’t want her to do the same thing to me. And she has asked. Nobody knows we’re having a boy except me and my husband and we plan to keep it that way. But my sister has asked what our boy name is and as an afterthought she asked for our girl name too when she realized I knew what she was getting at.

I refused to tell her. She tried to whine about name sharing being the fun part of pregnancy. I made up a couple of names on the spot to tell her and she saw through it. She told me to just tell her the name and I said no again.

She asked a few more times, she even asked in front of our family. It was our brother who joked that nobody should tell the baby name thief the name they’ve chosen. My sister got upset and asked if that’s why I wouldn’t tell her. She told me I was holding something against her that she never did to me. Mom asked why I couldn’t just share the name and everyone would know my husband and I chose it first if my sister used it but I still said no.

AITA?