My first thought was Sylvia I think that would be adorable as a little sister to Lillian, then I thought of:
I think these would all sound beautiful as the name for a wonderful little princess and as a little sister to such a well named big sister Lillian
I tried to go with classy yet fun names to match the spirit of Lillian
I started this thread originally but my password isn't working.
Thanks for all the replies! Lots of great suggestions to consider.