Heiress refers exclusively to females; males in similar circumstances are called 'heirs'. Although they both inherited, she's the heiress and her brother the heir.
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Sole Daughter
As an heiress, particularly in historic contexts, the woman often inherits as she is the only daughter. She, being the sole daughter, was the heiress of the family estate.
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Wealth Connotation
The term 'heiress' often implies significant wealth or property, not just any inheritance. The media dubbed her a wealthy heiress after the inheritance of the tycoon's fortune.