Elizabeth Jane is my pick. Lizzie Jane is cute, but I am not really a fan of Lizzie. There are so many other fabulous nicknames for Elizabeth.
I would also use a more daring middle name with a basic name like Elizabeth. Elizabeth Wren, Elizabeth Skye, Elizabeth True
Elizabeth is okay as a sister to Julia. They are both classic, but Julia has just a bit more je ne sais quoi.
Isabel is a variation of Elizabeth, so I like that one for you. I also much prefer Izzy over Lizzie.
However, unless you call Julia a nn, like Jules, her sister probably shouldn't have a name with a nickname.
Suggestions for Julia's sister: Rosalind or Rosalie, Adele or Alice