I'm a little confused, as you seem to have a few on your list (Grace, Alice, Mila, Vivienne, Morgan) that don't seem to have the same "rare" feel as Saffron and Adelais.
That said though, how about:
From your list, I see this as suiting what you want, a unique name:
Nice names, but definitely not unique:
Personally I dislike Grace. It's also very common, so I'd be surprised to see it in a sibset with Saffron or Adelais.
None of those names are as rare as Saffron and Adelais.
Ophelia, Allegra, Saskia, Suki and Avalon are uncommon and one of those would fit the bill. Grace, Alice, Mila, Millie, Vivienne, Sasha and Morgan are all too normal. Clover is pretty uncommon, but I don't like Clover and Saffron in the same sibset.
Names that seem to go with Saffron and Adelais would be rare British names, Something exotic and eccentric.