Violet would be my pick!
Gosh, this is hard.
Evelyn has really grown on me and I think it's got the most lovely vintage charm to it now, so she needs a similarly lovely sister name. "
Evelyn and
Violet" just sounds delightful to my ears. They are similar and have the V to sort of link them, but each has her own personality.
Vivian is another name I love, but "
Evelyn and
Vivian" are TOO harmonious, too close to rhyming. It would make the set sound like "
Sherri and
Terri" levels of matcy-matchy (which may be your thing).
Evelyn and
Josephine are really cute together too. Very vintage-cool indeed.
Other ideas:
Louisa -
LuluWilhelmina-
Billie,
Willa,
MinaAdeline -
AddieClementine -Clemmie
Helena - Helly,
LenaVirginia -
GinnyAnneliese -
AnnieEugenia -
Ginny,
GenieMiriam -
Mimi, Mir,
Miri
Anything but
Addison.
Even Alison would be better.