I love [name_f]Io[/name_f]! I always liked it as a (kind of stretchy) nickname for Violet/Viola.
[name_f][/name_f]I think [name_f]Io[/name_f] is lovely on its own, too, though. It’s definitely an unusual name and I think it is short and I’d personally be more likely to use it as a nickname, but I don’t think that it’s too short to be usable.
[name_f][/name_f]From your list, I like [name_f]Io[/name_f] [name_f]Selkie[/name_f] or [name_f]Io[/name_f] [name_f]Rosemary[/name_f].
[name_f][/name_f]Maybe
[name_f][/name_f][name_f]Io[/name_f] [name_f]Lillian[/name_f]
[name_f][/name_f][name_f]Io[/name_f] [name_f]Penelope[/name_f]
[name_f][/name_f][name_f]Io[/name_f] [name_f]Talitha[/name_f]
[name_f][/name_f][name_f]Io[/name_f] [name_f]Persephone[/name_f]
[name_f][/name_f][name_f]Io[/name_f] [name_f]Lenora[/name_f]
[name_f][/name_f][name_f]Io[/name_f] [name_f]Serena[/name_f]
[name_f][/name_f][name_f]Io[/name_f] [name_f]Delphine[/name_f]
Brothers for [name_f]Io[/name_f]: [name_f]Eden[/name_f], [name_m]Laszlo[/name_m], [name_f]Poet[/name_f], [name_f]Sailor[/name_f], [name_m]Ari[/name_m], [name_m]Sky[/name_m], [name_m]Yves[/name_m], [name_m]Remy[/name_m]
[name_f][/name_f]Sisters for [name_f]Io[/name_f]: [name_f]Eira[/name_f], [name_f]Zelda[/name_f], [name_f]Cleo[/name_f], [name_f]Ida[/name_f], [name_f]Rue[/name_f], [name_f]Lucia[/name_f], [name_f]Sable[/name_f], [name_f]Salome[/name_f]
[name_f][/name_f]I think it works well with your other names, but I do feel like it would fit a little better as a nickname. Maybe for a name with the initials IO? [name_m]Or[/name_m] something like [name_f]Iona[/name_f] or [name_f]Iolanthe[/name_f]. [name_m]Even[/name_m] by itself, I think it still fits, though. Your names all have a bit of a dark spacey vibe to me so it fits nicely.