She's a bit of an oddball and definitively less caring about others compared to Isa. Remember how she looked unconcerned upon seeing the guy Sephiroth burned? Things like that.
Although they care for each other, they don't act nor consider people the same way. It's not something so noticeable at first, but you'll eventually see the obvious gap between their way of thinking. (Which I planned to use as a way to build tension between the twins)
Anyway, I sincerely hope we can get the pair to reach the group soon, so that we can all move forward.