Love, love
Claire. It's known, but not overly used. And your brother can have a lot of fun with the mn if he wants to. My niece is named
Clare so I have a bias toward that spelling. The "i" in
Claire looks a tad fussy to me. But it's definitely the most common and therefore the easiest-to-live-with spelling of the name.
My niece is
Clare Siobhan. I have a stock combination
Clare Angelica. But I can also see a more understated (yet still elegant) mn for it like
Claire Evelyn or
Claire Elisabeth. The possibilites are endless with
Claire.