[name_m]Hi[/name_m] Berries!
Does [name_f]Catherine[/name_f] and [name_f]Caia[/name_f] work for siblings?
Thank you! 
Yes, I think so. I see them as pretty different, stylistically–that might bother some people who really want a matching sibset. On the other hand, some people avoid repeating the first letter/sound of names within a sibset, but to me it’s fine as long as they’re not confusingly similar. So yes, I think [name_f]Catherine[/name_f] and [name_f]Caia[/name_f] could be siblings.
I would say no, since it’s said like [name_f]Kaia[/name_f] which originated as a nickname for [name_f]Katarina[/name_f] which is a variant of [name_f]Catherine[/name_f]. So I feel like it would be sort of like using [name_f]Catherine[/name_f] and [name_f]Kate[/name_f] or whatever. But I think something similar like [name_f]Maia[/name_f] and [name_f]Catherine[/name_f] would work.
I know a [name_f]Catherine[/name_f] in Denmark who goes by [name_f]Caja[/name_f]/[name_f]Caia[/name_f] so it seems weird to me, but most people would never make the connection. The sounds of the names are nice together, to me. Although [name_f]Kaja[/name_f] and co. are diminutives of [name_f]Catherine[/name_f]/[name_f]Katarina[/name_f] and co., there are other sources for [name_f]Caia[/name_f] (from [name_m]Caius[/name_m]) and [name_f]Kaya[/name_f] (African name) so I guess you could choose a different root for [name_f]Kaia[/name_f]/[name_f]Caia[/name_f] so that both sisters have their own names with different meanings and etymological roots.
Too close for my taste, especially as [name_f]Caia[/name_f] is used as [name_f]Catherine[/name_f]'s nickname.
My first thought was that [name_f]Caia[/name_f] seems like it could be a [name_f]Catherine[/name_f]'s nickname. I don’t think they work for siblings.
yes it works
I don’t think the names are actually etymologically related at all, but seeing as how many people actually use some form of [name_f]Caia[/name_f] as a nickname for [name_f]Catherine[/name_f], I would say no.
No. [name_f]Catherine[/name_f] is beautiful and classic. [name_f]Caia[/name_f] seems trendy in comparison.