What would you name a sister for Mazzy?
Honestly, I prefer Maisie to Mazzy, but I totally understand your search for a unique name and Maisie is currently at #658. Sully is a really cute choice as well, an adorably unusual name for sure! Your name choices seem so unique and fresh to me 
Here are my humble suggestions:
My first suggestion is Sylvie. It does end in an ie, but it is very similar to your #1 boy name, Sully. It even means “from the forest” while Sully means “from the south meadow”. Sylvie and Mazzy sound quite striking together and are both unusual names that don’t rank in the top 1000.
My second suggestion is Calla. Calla is not in the top 1000 and means ‘beautiful’. It is simple and sweet like your other name choices and has a similar structure to Tedda and Pippa. Once again, Mazzy and Calla sound very nice together.
My next suggestion is Thea. Thea comes from the same root as Teddy, but sounds a little more charming. However, it is currently ranked at #776, possibly making it too popular for your taste. Mazzy and Thea sound absolutely darling together.
Finally, I suggest Juno. Juno is not in the top 1000 and means “queen of the heavens”. It is a little more spunky than June and fits in better with your #1 girl choice, Mazzy. You could still use the nickname June or even Juni.
I hope this helped, good luck!
[name_u]Perrin[/name_u]
[name_f]Tula[/name_f]
[name_f]Etta[/name_f]
[name_f]Freya[/name_f]
[name_u]Rue[/name_u]
[name_f]Sidda[/name_f]
[name_f]Harlow[/name_f]
[name_f]Zula[/name_f]
[name_f]Zinnia[/name_f]
[name_f]Briar[/name_f]
[name_f]Grier[/name_f]