See the results of this poll: Are Julia and Jillian too similar for sisters?
Respondents: 14 (This poll is closed)
- Yes - too similar : 10 (71%)
- No- they work fine together: 4 (29%)
I think its fine, as long as they aren’t called [name]Juli[/name] and [name]Jilly[/name] 
I do think [name]Julia[/name] and [name]Jillian[/name] are too close. While I (and the average person) wouldn’t put them together as both forms of [name]Julia[/name]/[name]Julius[/name]/etc. I do think they just have too many similar sounds. You could probably do [name]Julia[/name] ____ and ____ [name]Jillian[/name], for example, or [name]Jillian[/name] _____ and ____ [name]Julia[/name].
I think they’re fine together.
[name]Julia[/name] and [name]Jillian[/name] are too similar for my comfort level. They’re also worlds apart when it comes to style. [name]Julia[/name] is a [name]Roman[/name] name of great antiquity while [name]Jillian[/name] is more contemporary in vibe (especially the modern “J” spelling). The [name]Gillian[/name] spelling has been around since the sixteenth century.
I think the sounds are too similar to use but I get a completely different image when I think of a little girl named [name]Julia[/name] and a little girl named [name]Jillian[/name]. To be [name]Julia[/name] is a girly girl while [name]Jillian[/name] is more of a [name]Tom[/name] boy.