I like these names…
1 [name]Helena[/name] - a “natural” and more mainstream pairing with [name]Amelia[/name]
2 [name]Magdalena[/name] - works with [name]Amelia[/name] best
3 [name]Isolde[/name] - I think this goes well with [name]Persephone[/name] and [name]Guinevere[/name] but it would still leave out [name]Amelia[/name] as the only non-mythological name in your sibset.
4 [name]Leontine[/name] - Would you be pronouncing this name the French way (lay-ON-teen)? If you were going to pronounce it like the boys name [name]Leon[/name] (lee-on) then I would find it a bit matchy with the ending of [name]Amelia[/name] ([name]Lia[/name] and [name]Leon[/name]).
Names Like [name]Amelia[/name]: [name]Louisa[/name], [name]Clara[/name], [name]Eliza[/name], [name]Susanna[/name], [name]Caroline[/name], [name]Charlotte[/name]/[name]Carlotta[/name], [name]Cecily[/name], [name]Lucy[/name]/[name]Lucinda[/name], [name]Violet[/name], [name]Madeleine[/name], [name]Tabitha[/name], [name]Fiona[/name], [name]Bridget[/name], [name]Beatrice[/name]/[name]Beatrix[/name], [name]Philippa[/name], [name]Imogen[/name], [name]Jemima[/name], [name]Felicity[/name], [name]Christabel[/name], [name]Isadora[/name], [name]Corinna[/name], [name]Celeste[/name], [name]Estella[/name]/[name]Estelle[/name], [name]Juliet[/name], [name]Daniela[/name], [name]Serena[/name], [name]Vanessa[/name], [name]Miranda[/name], [name]Catherine[/name]/[name]Katharina[/name], [name]Francesca[/name], [name]Eleanor[/name]/[name]Leonora[/name], [name]Clarissa[/name], [name]Eudora[/name], [name]Florence[/name], [name]Rosalind[/name], [name]Rosamund[/name], [name]Roxanna[/name], [name]Rosalie[/name], [name]Theodora[/name]/[name]Theodosia[/name], [name]Frederica[/name], [name]Diana[/name], [name]Juliana[/name], [name]Sabrina[/name], [name]Justina[/name], [name]Marcella[/name], [name]Mattea[/name], [name]Tatiana[/name], [name]Veronica[/name], [name]Mirabel[/name], [name]Regina[/name], [name]Valentina[/name], [name]Desdemona[/name], [name]Larissa[/name], [name]Renata[/name], [name]Candida[/name], [name]Perdita[/name], [name]Clementine[/name], [name]Cosima[/name], [name]Decima[/name], [name]Delilah[/name], [name]Davina[/name], [name]Dorothea[/name], [name]Evelina[/name], [name]Gabriella[/name], Raphaella
Other Suggestions
Carmenta
[name]Charis[/name]
[name]Charmian[/name]
[name]Coraline[/name]
Deianira
Druantia
Edingel
[name]Eglantine[/name]
[name]Eirene[/name]
[name]Eluned[/name]
Endellion
Eucleia
Euphrasia
Everilda
Fidelma
[name]Finola[/name]
[name]Gaenor[/name]
[name]Galiena[/name]
Genista
Heidrun
Hespera
[name]Hypatia[/name]
Igraine
[name]Inanna[/name]
[name]Ismene[/name]
[name]Jacintha[/name]
[name]Jacquetta[/name]
[name]Jessamine[/name]
[name]Juniper[/name]
[name]Laetitia[/name]
[name]Lalage[/name]
[name]Leocadia[/name]
[name]Linnea[/name]
[name]Lorelei[/name]
[name]Lucasta[/name]
[name]Lumina[/name]
Lyonesse
[name]Macaria[/name]
Marpessa
[name]Maxima[/name]
Mélusine
Merula
Messalina
[name]Mireille[/name]
Nephthys
[name]Nerissa[/name]
Olympias
Osyth
[name]Ottoline[/name]
Pélagie
[name]Peninnah[/name]
[name]Persis[/name]
[name]Phaedra[/name]
Polyxena
[name]Pomona[/name]
[name]Reseda[/name]
[name]Rhonwen[/name]
[name]Romola[/name]
[name]Sanchia[/name]
[name]Sapphira[/name]
Séraphine
Sunniva
[name]Tanith[/name]
[name]Tanaquil[/name]
Tethys
[name]Thalassa[/name]
Theano
Theoclea
[name]Thetis[/name]
[name]Undine[/name]
[name]Vashti[/name]
[name]Verbena[/name]
[name]Viridis[/name]/[name]Viridiana[/name]
Zéphyrine