Arena isnt my cup of tea, but thats just my opinion. All i get in my head when i see it is "Tina Arena", maybe if it was spelt different.........i dunno? Arina, Areena, Arinah etc. Actually, spelling it different changes the dynamic of the name an enormous amount for me, how interesting. I dont mind it with those spellings.

Aria is gorgeous!! I dont think you should worry about your "friend" having a child called that at all and i think its rude of your sister to say its rude of you to use that name. You dont have contact with this person really so why cant you use the name if you love it? It would be different if this was your sisters name choice or your BFF's or something, but an aquaintance, not an issue i say.