If you had a child with a name that’s not in the top 1000 would you feel that baby #2 needs a name just as distinctive? Not really, but then if you had [name]John[/name], [name]Mary[/name] and [name]Cassiopeia[/name], it would look a little odd. Depends on the relative popularity of the names I think.
Does it matter less if they’re opposite sex children? No, it doesn’t matter less. Again, [name]John[/name], [name]Tom[/name], [name]Frank[/name] and [name]Dave[/name], followed by [name]Andromeda[/name], would seem a bit off.
If you met [name]Perdita[/name] & [name]Beatrice[/name] would you be thrown off? What about [name]Perdita[/name] & [name]John[/name]? [name]Wren[/name] & [name]Colette[/name]? [name]Perdita[/name] & [name]Beatrice[/name] go lovely together! [name]Perdita[/name] & [name]John[/name] is fine. [name]Wren[/name] & [name]Colette[/name] feels a little off because I feel like [name]Wren[/name] is modern, simple and free whereas [name]Colette[/name] is dated and a little stuffy- but that’s how they feel together, nothing to do with popularity.
Is it possible for styles to match/harmonize even if there’s a great inconsistency in the popularity of a pair of names? Yes! And some popular names just sit fine with anything. Take my beloved [name]William[/name], for example. He’s a fine match with [name]Sophia[/name], [name]James[/name] or [name]Elizabeth[/name] but I think he’d pair equally well with sub-1000 names, like [name]Rosalind[/name] and [name]Leopold[/name].
If you met [name]Perdita[/name], [name]Beatrice[/name] & [name]Catalina[/name] would [name]Perdita[/name] seem glaringly different? Nope. That’s a rather fantastic trio of sisters right there.
What if you met [name]Perdita[/name], [name]Beatrice[/name] & [name]Cornelia[/name]…would [name]Beatrice[/name] seem very plain & common? Or like the “normal” sister next to 2 more over-the-top choices? No. They still all fit well together in my eyes.
What if we put a 4th child in this sibset. A boy named [name]John[/name]. Is that OK cause he is the lone boy & style consistency matters less? I don’t think that the style shouldn’t matter just because he’s a boy. But I reckon [name]John[/name] fits well with everything 
Honestly, I don’t think popularity really matters with siblings. As long as you’re not planning on a sibset of [name]Aphrodite[/name], Alixxxsandruhh, Amhlaoibh and [name]Zak[/name], I think you’d be fine 