The only Robert Meldrum Dewar I have vaguely fitting this description was one born at Auchterarder, Perthshire 12/9/1900, but as far as I know, no twin sister. He had seven siblings.He died 1960 at Cupar, Fife and married Jessie Ann Pert Sim at Perth in 1932.
His parents were Robert Dewar and Mary Meldrum, both Fife people
He had 2 children. Robert b1933 and Anne Sim Dewar b1935