I have to go with [name_f]Fiona[/name_f]! [name_f]Violet[/name_f] is beautiful and very popular atm, but [name_f]Fiona[/name_f] is very underrated imo. [name_f]Willow[/name_f] and [name_f]Fiona[/name_f] are a wonderful sibling pair. Congratulations and best of luck to you and your family 
I think [name_f]Willow[/name_f] and [name_f]Fiona[/name_f] flow better than [name_f]Willow[/name_f] and [name_f]Violet[/name_f]. I think if u love the name [name_f]Violet[/name_f] and want to stick with the theme then go with [name_f]Violet[/name_f] and, with time, [name_f]Willow[/name_f] and [name_f]Violet[/name_f] will sound natural together.
I like both names, but prefer [name_f]Fiona[/name_f] with [name_f]Willow[/name_f].
[name_f]Willow[/name_f] & [name_f]Violet[/name_f] together are a bit too nature-y. They could be nicknamed Tree & [name_f]Flower[/name_f].
[name_f]Willow[/name_f] and [name_f]Fiona[/name_f]!
[name_f]Violet[/name_f] is lovely but goodness, [name_f]Willow[/name_f] and [name_f]Fiona[/name_f] are just absolutely breathtaking together!
I see the appeal of both, but I have a serious weakness for [name_f]Fiona[/name_f] and can’t use it myself, so I’m always recommending that others use it! [name_f]Fiona[/name_f] and [name_f]Willow[/name_f] sound complementary but still distinct from each other, which to me is perfect for sisters. [name_f]Violet[/name_f] and [name_f]Willow[/name_f] are both clearly nature names and feel a bit more theme-y together.