Is it kai - la or kay - la or kee - la? Actually, I think Kailah looks a bit like your sister's trying TOO hard to be unique - just my opinion, though. Some people like to make up names. ^_^
Also, stay away from Rielle. That is John Edwards' "other woman"s name, and no matter how you feel about Edwards, almost everyone agrees that she's a real piece of work. Not someone you would name your beloved baby after.
How about Willow? I think that's an interesting name.
ADDED: I see below that the name Kailah actually exists as a Hebrew name and is pronounce Kay-la, so now I'm particularly confused when you say it's normally spelled "Kyla." Kai is a traditional Japanese boys' name (rhymes with 'sigh'), but the female form of it is usually Umi or Mari.