So far my two sibling characters are named [name]Jack[/name] and [name]Nina[/name]. Although I love the name [name]Nina[/name], I’m not sure if it goes entirely well with [name]Jack[/name]. Last name is undecided and I would love to hear some suggestions!
Are they modern characters? If so, what year were they born? If I write about modern people I tend to check the top 100 names for that year to get a feel for names which were popular then : )
Yes, they are modern. Thank you for the suggestion! 
To me, [name]Jack[/name] seems classic but modern.
Classics:
[name]Caroline[/name]
[name]Alexandra[/name]
[name]Lucy[/name]
[name]Amelia[/name]
[name]Ruby[/name]
[name]Nora/name
Not classic, but works:
[name]Lacey[/name]
[name]Alexa[/name]/[name]Alexis[/name]
[name]Emma[/name]
[name]Autumn[/name]
[name]Melody[/name]
I agree, I don’t think [name]Jack[/name] and [name]Nina[/name] go together very well. Since [name]Jack[/name] is a friendly-sounding name, I’d recommend names that sound likeable.
[name]Emma[/name]
[name]Lucy[/name]
[name]Kate[/name]
[name]Natalie[/name]
Maybe even [name]Annika[/name].
I like [name]Lucy[/name] and [name]Jack[/name]. And [name]Annika[/name], maybe nn [name]Ani[/name]. [name]Jack[/name] and [name]Ani[/name]. Cute.
[name]Cate[/name]
[name]Frances[/name] ([name]Franny[/name])
[name]Chloe[/name]
Thanks guys! I really like the names [name]Ani[/name] ([name]Annika[/name]), [name]Nora[/name], [name]Chloe[/name] and [name]Amelia[/name].
[name]Jack[/name] and [name]Sophie[/name]! First thing that came to mind. [name]Jack[/name] and [name]Caroline[/name] is a very nice suggestion, as is [name]Jack[/name] and [name]Lucy[/name].
Can you describe the characters? It helps people get a better image of a character, not every name fits on everyone, you know.