Another thing that could be worth noting, aside from pp’s supposing she is having trouble getting pregnant - maybe she is younger, I don’t mean too young, but it is somewhat difficult to be the young one in the family, and have aspirations of parenthood, while watching the older ones get married and have babies and start to use up some good names. What if she’s not ready yet, should people get the names because they got to it before she could? I mean, within reason. She is only reserving one name, and you could just cross that off your list rather than be petty. Some people are ok having two kids in a family with the same name, and some don’t want to copy or be copied. I think it is normal to want to set your plans out ahead.
When, say, there are 8 cousins and the first one to start a family gets to use any name they want, and the next 2-3 cousins start to make their families (maybe big families), and before you know it, several of the names you love are being used, and the choices get slimmer. You might not even have the same taste, but many people do, and seeing your short list shortened for you might not feel so good if you are behind in the babymaking. Instead of rushing into pregnancy (or for that matter, a relationship and marriage), it might seem like a good idea to reserve a name before anyone else gets a chance to swipe it, or else see what you’re left with by the time it’s your turn. Nothing.
The difference would be if the name she reserved had been a top choice and you were just ready to settle on it and tell people. Could be a headache. At this point, let her have it, and choose something else. No need to be irked about it. The point of her letting you know in time was so that neither of you have to settle for a name you don’t love. Maybe her wording didn’t really manage to make that point, but she doesn’t want to pick a different name, and you still are deciding, and from what I gather, it’s not a name you were desperate to use anyway, but she doesn’t know what’s on your mind, she just doesn’t want to see it inadvertently swiped, so she claimed it. You can’t read her mind either.