sienne

In a sentence

• La voiture est sienne.

  The car is hers.

  Here, 'sienne' acts as a possessive pronoun indicating ownership by a female subject.

• Ce livre appartient à Juliette, il est sienne.

  This book belongs to Juliette; it is hers.

  In this sentence, 'sienne' again is a possessive pronoun referring back to Juliette, the possessor.

• Cette décision était sienne.

  This decision was hers.

  Here, 'sienne' is used to convey that the decision specifically belonged to a female subject described earlier.