It’s unusual, but I think it’s usable. [name_f]Cressida[/name_f] Bonas gives the name some familiarity. [name_f]Essie[/name_f] is a cute nickname option if you want something less formal to use when she’s younger.
I pronounce it with the emphasis on the first syllable: CRESS-id-uh
For siblings, I could imagine names that are classic but unusual, or mythological, or posh:
[name_f]Beatrix[/name_f]
[name_f]Caroline[/name_f]
[name_f]Clio[/name_f]
[name_f]Daphne[/name_f]
[name_f]Edith[/name_f]
[name_f]Isolde[/name_f]
[name_f]Lorelei[/name_f]
[name_f]Rosamund[/name_f]
[name_f]Sabine[/name_f]
[name_m]Dashiell[/name_m]
[name_m]Edmund[/name_m]
[name_m]Hugo[/name_m]
[name_m]Lucian[/name_m]
[name_m]Maximus[/name_m]
[name_m]Rufus[/name_m]
[name_m]Rupert[/name_m]
[name_m]Sebastian[/name_m]
[name_m]Titus[/name_m]
I might avoid Shakespearean names though… (Unless you’re going for a theme, which could be cool!)