Mildred (imo suits your existing names to a T - and if you’ve watched the classic 1986 Worst Witch she has a wonderfully sweet namesake!)
Ivy (may be too popular, although I’d guess it was on par with Elise)
Florence (same as above)
Ingrid (popular in scandinavian countries but I presume not if you live in an English speaking country)
Harriet (not 2 syllables but fits your siblings set incredibly well IMO and ticks all your other requirements)
Marilyn (again, not 2 syllables but does work so well with your existing names)
Margot (again suits your existing names and ticks a lot of the boxes, but maybe too popular for you personally)
Alba
Thelma
Wilma
Xanthe (pronounced Zanthe - so doesn’t meet the “easy to spell” requirement unless you are Greek. But again I feel like it otherwise matches your siblings set very well in vibe, so suggesting in case it’s of interest)
Roxanne
Freya
Phoebe
Lucy (maybe too popular depending on where you live)
[name_f]Elise[/name_f] and [name_f]Greta[/name_f] are a lovely pair! I picture their sister being:
[name_f]Blythe[/name_f]
[name_f]Maude[/name_f]
[name_f]Ruth[/name_f]
[name_f]Jean[/name_f]
[name_f]Lorna[/name_f]
[name_f]Leona[/name_f]
[name_f]Oona[/name_f]
[name_f]Tilda[/name_f]
[name_f]Ruth[/name_f]
Ines/Inez
I second the suggestion of Margo/Margot, [name_f]Lena[/name_f], [name_f]Petra[/name_f], [name_f]Daphne[/name_f], [name_f]Zelda[/name_f], [name_f]Ingrid[/name_f]