We use
Sissie a lot at our house; it's a common Texas nn for any sister (as Bubba is for brother). The form most often used is Sissie-belle. (I like this -ie spelling to distinguish it from one who
is a sissy, in the sense of the mild insult). And I like to use
Fifi on
Sophie (with So
phia O
phelia Sera
phina, imo it's legit), though no one else does. I also like Bibi for Vivian/Bibiana, and ZiZi is my preferred nn for my combo
Zoe Zenobia.
But of all the cutesy nn, my favorite is
Coco, used in my case for
Socorro.