Re: Jillian, Julie, Julia, & Juliet.
I prefer Gillian to Jillian, though I far prefer Jill to Gill. I would enjoy Gillian and Juliet as mns for sisters, preferably twins, but I wouldn't use them as fns because they would inevitably get shortened.
Julia is not a name I've eve warmed to, though I've known some nice ones, and Julie is plain and beige and boring.