Forbes was born the third and youngest daughter of John Forbes-Sempill, 18th Lord Sempill and 9th Baronet in 1912, christened Elizabeth Forbes-Sempill and known to the family as "Betty". Betty was raised female, but with a childhood dominated by a widespread gender insecurity; after refusing to go away to a girls' school, she was educated at home, and the culmination of this unhappy upbringing was coming out as a debutante in London in the late 1920s. Betty was promptly sent abroad by the family, studying at Munich University, before returning to Scotland and leading a troupe of Scottish country dancers - the "Dancers of Don".