This thread has happened sooo many times in reverse,the first thing I thought was your SIL on another forum when you announced your DD's name:

SIL: I'm so upset, my SIL just said she's naming her daughter X Y, I've loved that name for a girl since I was 5 years old.

Others: Are you sure it's the same name, you should check.

SIL: I checked, it is.

Others: Just go ahead and use it anyway, are you really close to her? Do you see her all the time? I think it'd be a really nice bond between cousins, she should take it as a complement, nobody owns a name etc etc.

that was my first (silly) thought