I voted for [name_f]Brynn[/name_f], [name_f]Niamh[/name_f] & [name_f]Violet[/name_f] for a couple reasons. 1. I like them all individually, along with a bunch of the other names on your list. 2. I think they sound right with Pieternella (as do some of the others). 3. I think they’re strong enough to work beside [name_f]Savannah[/name_f] without being too similar. I could imagine sisters [name_f]Savannah[/name_f] & [name_f]Brynn[/name_f], even though they’re quite different.
I really like [name_f]Elsie[/name_f] and think it works with MN but to me, doesn’t sound substantial enough to be [name_f]Savannah[/name_f]'s sister. [name_m]How[/name_m] about [name_f]Eliza[/name_f] nn [name_f]Elsie[/name_f]?
I think [name_f]Hazel[/name_f] works, but I just don’t love the name.
[name_f]Vivienne[/name_f] I also think works, but I don’t think you can avoid [name_f]Viv[/name_f] / [name_f]Vivi[/name_f] her whole life. [name_f]Vivienne[/name_f] is bound to get shortened, and the most intuitive nn is [name_f]Vivi[/name_f] / [name_f]Viv[/name_f].
I saw in your signature [name_f]Rose[/name_f] Pieternella. This name looks perfect to me, and I think it fits with [name_f]Savannah[/name_f]. Is it not an option? I also like [name_f]Amelia[/name_f], [name_f]Isla[/name_f] or [name_f]Bryony[/name_f] from your signature.
I think that [name_u]Eden[/name_u] & [name_f]Violet[/name_f] work nicely with both [name_f]Savannah[/name_f] and Pieternella, so they got my votes. I also think [name_f]Elsie[/name_f] & [name_f]Ivy[/name_f] work quite well with both.