I love how everyone is so logical on here... My sister gave me her shortlist of 6 names before she was even married. I was not to touch any of her names. She had her daughter 3 months before I did and chose the 2nd girl name on her list.
When my daughter was born I made sure to stray from her choices, and my OH and I didn't even settle on a name until the day we left the hospital. My sister blew a gasket because I used the same middle name as the 1st name on her list.
She was soooo mad that she then accused me of getting pregnant to steal her limelight even though we had discussed that we were both going to be trying around the same time....
Anyway... I hope you have a good relationship with your sister, and just be honest with each other!