This is the 4939th most frequent French word.
sienne
"Sienne" means "his/hers/its own" (feminine singular possessive pronoun).
Here, 'sienne' acts as a possessive pronoun indicating ownership by a female subject.
La voiture est sienne.
The car is hers.
In this sentence, 'sienne' again is a possessive pronoun referring back to Juliette, the possessor.
Ce livre appartient à Juliette, il est sienne.
This book belongs to Juliette; it is hers.
Here, 'sienne' is used to convey that the decision specifically belonged to a female subject described earlier.