Briar and Casimir are two different genres, I can't see them as siblings. Casimir / Kasimir has such an Eastern European air to me (I've met a few guys named Kazimierz, living in a highly Polish area) and is a bit cosmopolitan, whereas Briar is very Western and down to earth.
That said, while Casimir is not a name I'd really consider using, I quite like Briar.
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