Sandra Brown - The Switch book description:
Identical twins Melina and Gillian Lloyd haven't considered switching places since childhood.
So when Melina proposes that Gillian take her place as a media escort to NASA astronaut Col. "Chief" Hart, she refuses...at first. The following morning Melina receives terrible news: her sister has been brutally murdered - and Chief, though innocent, is the prime suspect.
He and Melina are determined to find the killer, a megalomaniac whose horrific schemes require Gillian's replacement, her identical twin - Melina.