Hmm… Assuming we’re allowed to go off-script (or off-UC), I think [name_f]Cassiopeia[/name_f] would be just the perfect sister for your boys.
Long, extravagant but not outrageous, and with a long and interesting history. I like that she’d have her own initial and that all of their names would share a subtle sound connection with that “io” sound. [name_f]Io[/name_f] would actually be a cool nickname too, but [name_f]Cassiopeia[/name_f] gives you plenty of scope for creative nicknaming. Plus, there’s something of that [name_f]Chrysanthemum[/name_f] about [name_f]Cassiopeia[/name_f] to me… 
If it were me, I’d stick to a single middle, to match your boys’ names. [name_f]Cassiopeia[/name_f] [name_f]Helen[/name_f] or [name_f]Cassiopeia[/name_f] [name_u]Jule[/name_u] would be lovely. I also think [name_f]Cassiopeia[/name_f] [name_u]Vesper[/name_u] would be a beautiful mix of light and dark 