Oh, I think all three names are beautiful (the gemstones as middle names is a sweet unifying factor, too).
I don’t think they’re confusingly matchy, but I do like that you are trying to make sure each twin has a distinctive name–as someone who has twins in the family I think this is VERY important.
So, ultimately, I think [name_f]Willa[/name_f] and [name_f]Stella[/name_f] are not too matchy (and not at all so with [name_f]Tessa[/name_f]), but I would probably try a few alternatives out for [name_f]Stella[/name_f] (since [name_f]Willa[/name_f] is the family name–I also [name_u]LOVE[/name_u] this name) and see if there’s a less similar sound pattern that sticks. If so, all the better, if not, I would go ahead and use [name_f]Stella[/name_f].
[name_f]Estelle[/name_f]
[name_f]Tessa[/name_f], [name_f]Willa[/name_f], and [name_f]Estelle[/name_f] sounds great to me, and a benefit here is that [name_f]Stella[/name_f] could still be a nickname for [name_f]Estelle[/name_f]. You could even do [name_f]Tessa[/name_f], [name_f]Willa[/name_f], and [name_f]Estella[/name_f]. I know some people have a preference for the same number of syllables for their children, but I think introducing a three syllable name [name_f]Estella[/name_f], would make it stand out more from [name_f]Willa[/name_f] and [name_f]Tessa[/name_f].
[name_f]Sonia[/name_f], [name_f]Sophie[/name_f], [name_f]Sylvie[/name_f]
Other lovely two-syllable “s” names that have a similar vibe to me. [name_f]Sophie[/name_f] and [name_f]Sylvie[/name_f] could be changed to [name_f]Sophia[/name_f] or [name_f]Sylvia[/name_f], of course, if you want to go for a three-syllable name instead.
[name_f]Luna[/name_f], [name_f]Celeste[/name_f]
Have the same dreamy astrological vibe as “[name_f]Stella[/name_f]” to me
[name_f]Dora[/name_f], [name_f]Nora[/name_f], [name_f]Cora[/name_f], [name_f]Flora[/name_f]
These names have the two-syllable, “a-ending” pattern, and a similar vintage-but-of-the-moment vibe to me, but the “ra” ending keeps them distinctive from [name_f]Willa[/name_f] and [name_f]Tessa[/name_f].
[name_f]Esther[/name_f], [name_f]Etta[/name_f]
Also vintage come-back names–though maybe a little heavier on the vintage vibe than [name_f]Stella[/name_f]–with similar sounds