Sibling Rivalry — Catherine Carson

Ruth Carter had always felt she was considered the pale shadow of her sister Stephanie, but when she meets Scott Davidson, her sister's boyfriend, she feels an attraction which threatens to disrupt her safe existence as a collator of antiquities in the local Museum.
The story is told in a series of flashbacks by some of the guests at the evening celebrations of a family wedding; but which sister will be with Scott as the last dance is announced?