Congratulations 

You have a lovely shortlist here are my thoughts on your names
[name_f]Joanna[/name_f] ~ fun, spirited and bright I think [name_f]Joanna[/name_f] is lovely with [name_f]Lydia[/name_f] she has this nostalgic 1950s selling cloudy lemonade at a lemonade stand on a warm summers day. [name_f]Joanna[/name_f] & [name_f]Lydia[/name_f] work nicely together despite [name_f]Joanna[/name_f] having a slighter different feel to [name_f]Lydia[/name_f].
[name_u]Julianne[/name_u] ~ personally I’m not overly keen on [name_u]Julianne[/name_u] she feels a little tired and I just prefer other names with a similar sound like [name_f]Juliet[/name_f] or [name_f]Julia[/name_f]. I would prefer [name_f]Juliana[/name_f].
[name_u]Zoe[/name_u] ~ sleek, visually attractive with the zippy z sound and feels modern. But isn’t what I would pair with [name_f]Lydia[/name_f].
[name_f]Rachel[/name_f] ~ I think [name_f]Rachel[/name_f] is just timeless a lovely, understated biblical name that works so well with [name_f]Lydia[/name_f] despite having slightly different vibes. Honestly [name_f]Lydia[/name_f] & [name_f]Rachel[/name_f] is lovely.
[name_f]Genevieve[/name_f] ~ I adore [name_f]Genevieve[/name_f] beyond elegant, has a lovely European flair, looks visually gorgeous, is very romantic and I love the nickname [name_f]Evie[/name_f]. Despite loving [name_f]Genevieve[/name_f] I do feel that [name_f]Genevieve[/name_f] doesn’t has the right feel for [name_f]Lydia[/name_f] sister. I think [name_f]Genevieve[/name_f] is better suited to a longer, slightly more romantic option something like [name_f]Evangeline[/name_f].
[name_f]Rosalie[/name_f] ~ I love [name_f]Rose[/name_f] and all very magical variants including [name_f]Rosalie[/name_f]. [name_f]Rosalie[/name_f] is beautiful, romantic has a lovely European flair and looks so pretty but similarly to [name_f]Genevieve[/name_f] I think she has the wrong vibe for Lydia’s sister. Actually [name_f]Genevieve[/name_f] & [name_f]Rosalie[/name_f] would make a lovely sibling set but for you I would take off your list.
[name_f]Annabel[/name_f] ~ even though on paper [name_f]Annabel[/name_f] feels frilly and romantic so in my head shouldn’t work as Lydia’s sister I actually think she works wonderfully. [name_f]Annabel[/name_f] just has the same feel to [name_f]Lydia[/name_f]. A lovely pairing.
[name_f]Tabitha[/name_f] ~ YES!!! She’s your sparkly gem the absolute crème de la crème of your list honestly [name_f]Tabitha[/name_f] is the perfect name for you despite the flow being a little off with [name_f]Cynthia[/name_f]. [name_f]Tabitha[/name_f] is just so wonderfully matched with [name_f]Lydia[/name_f]. They both have the same feel. [name_f]Tabitha[/name_f] just like [name_f]Lydia[/name_f] is biblical, underused, elegant, tailored, feels spunky, has the fun ‘a’ ending and is so classic. [name_f]Tabitha[/name_f] is a wonderful sister name for [name_f]Lydia[/name_f].
[name_f]Phoebe[/name_f] ~ a lovely hidden of gem of a bible name that does sound lovely with [name_f]Lydia[/name_f]. [name_f]Phoebe[/name_f] & [name_f]Lydia[/name_f] do feel very cohesive and fashionable. Great pairing. I think works very well with [name_f]Cynthia[/name_f].
[name_f]My[/name_f] favourite is [name_f]Tabitha[/name_f] followed by [name_f]Phoebe[/name_f] & [name_f]Annabel[/name_f] 
[name_f]Hope[/name_f] this helps