A sister to Madeline (long 'i').
Have I mentioned that you've got great taste! ;-)
I love Lucinda and I really would've considered it for Madeline but dh scrapped it right away.
Love Madeline (especially with long i)
For her sister, I like several on your list but my favorites are Eloise and Juniper.
Eloise and Juniper are definitely faves of mine.
If I could name another daughter today it would be Gwendolyn Thea K_________(3-syllables).
For some reason I'd say Delilah.