I like a lot of the names mentioned above, and I added some of my own
Hope this helps! Good luck! Lillian is such a pretty name, I'm sure her sisters name will be just as pretty!
I have a daughter named Lillian Grace, age 7. My sons are named Jack William, 9 and Miles Henry, 3. We are trying to name our next baby daughter who is going to be born any day. I'll share my short list of favorites, maybe it will help. It really depends on your last name. With your three + one syllable formula it makes me suspect that your last name is two syllables like ours. My top name is Camilla Mae. I think it is absolutely perfect to go with Lillian and it sounds great with our last name. Heartbreakingly, my husband still doesn't like it so we will have to use something more common that doesn't have the same great ring to it. I would have avoided the nn Cami (dated) and use Milla, esp if you call your daughter Lily. If you don't, Calla could work as well for a nn. Or Callie. Some of my other top names for a sister for Lillian are Vivian, Marilyn, Rose or Rosemary, Gloria nn Glory, Marguerite, Elise, Maxine, and Violet. I wish you luck! Maybe you will get to use one of the great names that I won't get to :(