Of first names, I like [name_f]Veda[/name_f], [name_f]Sadie[/name_f] and [name_f]Opal[/name_f] best with [name_f]Willa[/name_f].
My favorite combos with those names are [name_f]Veda[/name_f] [name_f]Rose[/name_f], [name_f]Veda[/name_f] [name_f]Margot[/name_f] (though I think it sounds a bit solemn next to [name_f]Willa[/name_f] [name_f]Mae[/name_f]) and [name_f]Sadie[/name_f] [name_f]Adelaide[/name_f].
I also like [name_f]Tallulah[/name_f] [name_f]Rose[/name_f] with [name_f]Willa[/name_f] [name_f]Mae[/name_f], but [name_f]Willa[/name_f] and [name_f]Tallulah[/name_f] might be too close in sound.
[name_f]Willa[/name_f] and [name_f]Lola[/name_f], and [name_f]Willa[/name_f] and [name_f]Winnie[/name_f] seem too close to me. My favourite is [name_f]Sadie[/name_f] but [name_f]Sadie[/name_f] [name_f]Adelaide[/name_f] rhymes a bit so I like [name_f]Sadie[/name_f] [name_f]Adeline[/name_f] or [name_f]Sadie[/name_f] [name_f]Rose[/name_f] better. [name_f]Ivy[/name_f] would also be a nice sister name to [name_f]Willa[/name_f].
[name_f]Opal[/name_f] [name_f]Adelaide[/name_f] really stood out to me. I think that is a beautiful name and a great sister for [name_f]Willa[/name_f].