OP, I'm just repeating others when I say the most amusing thing about this is that YOU first said you were thinking of using the name. It's like you're holding a candy bar, she yanks it from your hand, then insists you can't touch HER candy!
Charlotte is a beautiful name. I have an Aunt (in her 60s) and a SIL (in her 40s) with that name; it seems to be a timeless name without having been over-used.
When it comes to name-stealing, of course I think it isn't even possible, but I completely appreciate the claim if the name is really unusual.
My son is 9 and I didn't know anyone at all with his name when he was born ("Levi", which may be uncommon, but not at all unusual). I was very happy about that. Now his name is growing steadily in popularity and I'm a bit disappointed because I liked it being unique. I never would have tried to prevent a loved-one from using it on their own child, though!
I wonder how a person would feel if someone had a unique naming approach that someone 'stole' rather than the specific name (like "I'm going to name all my daughters after Disney princesses... Tianna, Ariel, and Belle), then someone else, like her SIL, comes along and names her kids Jasmine and Aurora. I can imagine some people getting really territorial!