I am due next week with my third child. My husband and I have 2 boys and we are waiting until the birth to find out the sex of our baby. We finally settled on a boy's name and a girl's name this week. We have never discussed our name choices until the birth in the past, but since it is so close we answered when asked by close family. My husband's sister got very upset when she heard our girl choice because she said it was the name she had picked out for her daughter (we actually chose this 7 years ago, had our first child been a girl). She is married, but has no children and is not even pregnant yet. Of note, my sister-in-law and I have the same name and the baby name in question is very similar to our name. It has become rather uncomfortable because she is very upset about this. I feel we chose this name independently and have the right to name our child whatever we want. Any thoughts?
This reminds me of a couple of cousins of mine, sisters, who ended up being pregnant with girls at the same time and were literally racing each other to the finish line because they both wanted to use the same name. One gave birth a few weeks early and got the name. Her sister had to settle for her second choice. I think it's still a sore point between them. I really disliked the name they were fighting over anyway.
I hope your SIL will just get over it. She's not even pregnant yet. You guys may end up with a boy, and so may she. Nothing's stopping her from using the same name some day if you both have girls (I have two other cousins with the same first name and it's never bothered anyone), and she can always pick a different name if she doesn't want her kid to share it with an older cousin. There are lots of beautiful names out there.
Of course you have the right to name your baby whatever you've chosen. Maybe her reaction was related to something deeper, like resentment and frustration because you've already got children and she's still waiting. In my experience it's a hard, hard thing to watch your siblings having kids when you are trying or trying to convince your spouse to try or waiting to find a spouse so you can start trying...
Best wishes to you as you get ready to meet your new baby, whatever his or her name will be!