Same vibe, greek, mithological, unusual…
Hermione
I love [name_f]Titania[/name_f] and [name_f]Olympia[/name_f], both very powerful and great names for girls 
Pandora
[name_f]Antigone[/name_f]
[name_f]Athena[/name_f]
[name_u]Artemis[/name_u]
Love [name_f]Olympia[/name_f]! [name_f]Olympia[/name_f] [name_u]Wren[/name_u] has been on my list for a bit now!
[name_f]Aurora[/name_f]
[name_f]Elowen[/name_f]
[name_f]Cornelia[/name_f] or [name_f]Cordelia[/name_f]
Olympia and…
Demeter
[name_f]Persephone[/name_f]
[name_f]Euridyce[/name_f]
[name_f]Circe[/name_f]
[name_f]Ianthe[/name_f]
[name_f]Aglaia[/name_f]
[name_f]Leandra[/name_f]
Olympia and [name_f]Atalanta[/name_f].
Olympia and [name_f]Xanthe[/name_f].
Hermione
[name_f]Titania[/name_f]
[name_u]Calypso[/name_u]
[name_f]Circe[/name_f]
[name_u]Artemis[/name_u]
[name_f]Athena[/name_f]
[name_f]Persephone[/name_f]
[name_f]Pandora[/name_f]
[name_u]Tempest[/name_u]
[name_f]Juno[/name_f]
[name_u]Hero[/name_u]
I know it’s not really in the criteria (sorry 
) but I really like [name_f]Olympia[/name_f] and [name_f]Senara[/name_f].
For the longest time I had [name_f]Olympia[/name_f] and [name_f]Penelope[/name_f] as twin/sister names on my list (nn [name_f]Pia[/name_f] and [name_f]Nell[/name_f]). I still feel like [name_f]Penelope[/name_f] is a great choice for a sister to [name_f]Olympia[/name_f].
Other sister ideas:
[name_f]Persephone[/name_f]
[name_f]Hermione[/name_f]
[name_f]Phoebe[/name_f]
[name_f]Eugenia[/name_f]
[name_f]Helena[/name_f] / [name_f]Helen[/name_f]
[name_f]Artemisia[/name_f] / [name_u]Artemis[/name_u]
[name_f]Athena[/name_f]
[name_u]Calypso[/name_u]
[name_f]Electra[/name_f]
[name_u]Phaedra[/name_u]
[name_f]Theodosia[/name_f]
[name_f]Cressida[/name_f]
[name_f]Ariadne[/name_f]
[name_f]Andromeda[/name_f]
[name_f]Cassandra[/name_f]
[name_f]Pandora[/name_f]
[name_f]Ambrosia[/name_f]
[name_f]Megara[/name_f]
[name_f]Thalia[/name_f]
[name_f]Atalanta[/name_f]