[name_u]Jem[/name_u] [name_m]Dashiell[/name_m] is such a handsome name!
My favorites from your list:
[name_f]Lumi[/name_f]: [name_f]Lumi[/name_f] is a beautiful name. It’s short and sweet, but it has so much personality in just four letters. I love the “snow” meaning, but [name_f]Lumi[/name_f] feels like a very warm and yellow name to me. I think it complements [name_u]Jem[/name_u] quite nicely.
[name_f]Olive[/name_f]: [name_f]Olive[/name_f]'s (I believe) a little more popular than your other choices, but I don’t know if that would bother you or not. [name_u]Jem[/name_u] and [name_f]Olive[/name_f] sounds like a perfect sibset.
[name_f]Fern[/name_f]: I’ve loved [name_f]Fern[/name_f] ever since the little girl in [name_f]Charlotte[/name_f]'s [name_m]Web[/name_m]. It’s a vintage nature name that doesn’t seem to have caught on like [name_f]Ivy[/name_f] or [name_f]Iris[/name_f] have, and ferns are beautiful plants.
[name_f]Cosima[/name_f]: Both [name_u]Jem[/name_u] and [name_f]Cosima[/name_f] share the “m” so I think this sibset, while a little eccentric, could work really well! [name_f]Cosima[/name_f] is growing on me, it feels very starry and airy which is a nice complement to the more grounded [name_u]Jem[/name_u].
[name_f]Isolde[/name_f]: [name_f]Isolde[/name_f] is my favorite of your choices, but my least favorite as a sibling name to [name_u]Jem[/name_u]. However, I tend to gravitate towards more matchy sibling names- take my advice with a grain of salt! [name_f]Isolde[/name_f] feels so strong and feminine, like a warrior name.
[name_f]Mavis[/name_f], [name_f]Neva[/name_f], and [name_f]Zelda[/name_f] are also recommendations of mine. They go very nicely with [name_u]Jem[/name_u].
I like [name_f]Nell[/name_f], [name_f]Gwen[/name_f], and [name_u]Wren[/name_u], but I don’t like the way they sound with [name_u]Jem[/name_u]. Especially [name_u]Wren[/name_u]- I know the two names don’t rhyme, but they sound a little too close for my comfort.
[name_f]Daphne[/name_f] feels a little unbalanced next to [name_u]Jem[/name_u], but it’s a beautiful name. Maybe as a middle name for one of your shorter name options? (I actually really like [name_f]Daphne[/name_f] as a middle name. It’s really close in sound to [name_m]Dashiell[/name_m], which might be a nice way to connect the siblings’ names.)
With [name_u]Echo[/name_u] and [name_u]Hero[/name_u], I’m unsure about usability. [name_u]Echo[/name_u] is beautiful, but it also sounds like easy material for kids to tease. And [name_u]Hero[/name_u] might be mistaken by non-name nerds as another virtue or “high expectations” name, similar to [name_m]King[/name_m] or [name_m]Saint[/name_m]. I do like the name, but I would suggest [name_f]Hermione[/name_f], nn [name_u]Hero[/name_u], instead.
[name_f]Cleo[/name_f] would also be perfect, and feels much more traditional to me than [name_u]Echo[/name_u] or [name_u]Hero[/name_u]. What is your husband’s objection?