Girls favourites - Primrose, Athena, Darla
Boy favourites - Sacha, Sonny and Jesse (but not Sonny if you want unusual)
Penelope - Love this, I don't think it maters about your sister at all.
Indigo - It's ok, but almost obviously unusual if that makes sense?
Primrose - gorgeous name, I love it, beautiful happy spring flower too.
Callie (Calliope?) - I like Callie, detest Calliope, sounds to 'try hard'
Darla - American girl who works in a diner, but I kind of like it! Definitely wouldn't be any others in her class. It's a bit mad but I love it!
Vivienne - still old fashioned to me, I'd imagine a formidable lady in her late 50's.
Athena - gorgeous
Malin - sounds like a fish, I think the fish is Marlin.
Lilith - I associate this with an unpleasant TV character. But also Lilly is fairly popular, so not good if you are looking for an unusualy name as it'll get muddeld in to the Lilly's and Ellies possibly.
Rosaline - nice, sounds shkespearane and I much prefer it to Rosalind which I find quite clunky
Evander - Hollyfield? Who was he? A boxer? Might have made that up. It's ok.
Koa - la, I can only see Koala when I see this, not keen, bit silly I think,
Sonny - I love this name a lot but I thin it is fairly popular right now.
Indy - I like this but I associate it more with girls.
Sacha - Really like
Ivah - Not sure how to pronounce this, obviously if I know you and your child I'd learn
Marley - a dog, and a movie it is impossible to watch without becoming a blubbering mess. Also a really naughty boy where I live!
Jesse - Love this.
I have a normal first name but I had a bonkers surname and I loved it growing up, so many people on mumsnet say don't give weird names because you'll set them up for a lifetime of hardship and misery - it's really far from always the case! My sons first name is quite rare too, and he loves it.
Just my thoughts, good luck and congratulations.