Why has Lindsey Graham’s sister inherited his Senate seat after his death?

Three days after the sudden death of Republican Senator Lindsey Graham, his sister, Darline Graham Nordone, was sworn in on Tuesday to fill his vacant Senate seat at the suggestion of US President Donald Trump. South Carolina Governor Henry McMaster appointed Nordone, introducing her as Graham's 'darling little sister' who would 'finish his work for him now'. Graham, an influential senator, was a strong advocate for Israel and a hardliner on Iran. Darline, active in local government but with no elected political experience, inherited the seat because Graham died unmarried and without children.