I know a couple of Nellas (both short for Fenella) and they have sisters called Jemima (Mima) and Delphi.
If your Nella is a Fenella then you could go with another unusual name with more unusual nickname combo like:
Addie (Addison)
Vannah (Savannah)
Mimi (Miranda)
Ama (Amabelle)
Veta (Elisaveta)
Mika (Tamika)
Callie (Calliope)
Rory (Lorelai)
Vivi (Vivienne)
Sephy (Persephone)
Etta (Nicolette)
Or some standalone unusual names:
Amberley, Averil, Bronte, Blue, Bluebelle, Carissa, Cherry, Calista, Danika, Dulcie, Emery, Fallon, Fabia, Flavia, Giselle, Honey, Idina, Ianthe, Jessamy, Jerica, Julissa, Jona, Kimora, Karenza, Katinka, Larissa, Livia, Mala, Maeve, Mercy, Odette, Posy, Paloma, Pandora, Rosina, Robina, Rowena, Saffron, Sorrel, Taryn, Ulrika, Vashti, Veda, Xenia, Ysanne, Zeryn