Considering this for a daughter (who hopefully isn't too far off in the future), as it honours two great grandparents and two grandparents. If her name is shortened to a nn, it'd likely be Lillie or Lilly/Lillie-May.
What would you name a sister to Lillian? Really liking Dexter William Grant for a boy.
So far for sisters with Lillian, I've thought up:
Clara
Isobel
Vera* (Lillian Vera was my great grandmother's name, is it weird to use Vera as well?)
Pearl
Josephine
Eleanor
Evelyn
Cecily
Eliza
Florence
Catherine
Beatrice
Adelaide
Ada
Dorothy
Iris