[name_f]Adelaide[/name_f] is lovely!
Honestly I really love [name_f]Adelaide[/name_f] she’s warm, sunny, elegant, strong, has the lovely noble meaning and the sweet nickname [name_f]Addie[/name_f]. I completely understand why you love [name_f]Adelaide[/name_f] 
I think [name_f]Adelaide[/name_f] would work gorgeously with a [name_f]Lavinia[/name_f]. I think that sibling set has this overall [name_m]Roman[/name_m] Empire esque feel despite [name_f]Adelaide[/name_f] not being from this era this sibling set just feels a strong, powerful ancient vibe.
As I said above I think [name_f]Adelaide[/name_f] feels warm, sunny, elegant and strong. I think she’s a little unexpected and just has this strong, academic feel whilst reeking of summer.
[name_f]My[/name_f] favourite would be [name_f]Gwendolen[/name_f] for a very classical vibe and [name_f]Saskia[/name_f] for a more bohemian flair. Whilst with the boys I like [name_m]Holden[/name_m] with [name_f]Adelaide[/name_f] it creates a very southern feel. Others I think would work from your UC would be [name_m]Lysander[/name_m], [name_m]Faraday[/name_m] + [name_m]Casimir[/name_m] for boys. For girls I think [name_f]Eulalie[/name_f], [name_f]Viola[/name_f], [name_f]Cressida[/name_f] + [name_f]Leonie[/name_f].
[name_f]My[/name_f] personal sibling suggestions would be
[name_f]Clarissa[/name_f], [name_f]Susanna[/name_f], [name_f]Felicity[/name_f], [name_f]Freya[/name_f], [name_f]Penelope[/name_f], [name_f]Theodora[/name_f], [name_f]Leonora[/name_f], [name_f]Cecilia[/name_f], [name_f]Roberta[/name_f] & [name_f]Scarlett[/name_f]
[name_m]Willoughby[/name_m], [name_m]Peregrine[/name_m], [name_m]Vincent[/name_m], [name_m]Casper[/name_m], [name_m]Tobias[/name_m], [name_m]Gideon[/name_m], [name_m]Isaiah[/name_m], [name_m]Abraham[/name_m], [name_m]Sullivan[/name_m] & [name_m]Evander[/name_m]