Princess Margaret was the only sibling of Queen Elizabeth II.
Princess Margaret of York was born on August 21st, 1930 in Scotland in Glamis Castle, the ancestral home of her mother's family. At the time, Margaret's grandfather, George V, was the British King. Her father, who later became King George VI, was the younger brother of Edward VIII and ranked second in the line of succession to the throne. After Edward VIII's abdication because of his marriage to Wallis Simpson in 1936, her father surprisingly became king. This put her second in the line of succession, right after her older sister Elizabeth.
In the first half of the 1950s, Princess Margaret had a romance with Colonel Peter Townsend. He was divorced, meaning at that time according to public opinion, a royal wedding was out of the question.
At the time she would have needed her sisters approval to marry, which she didn't receive. Elizabeth asked her sister to wait for a year and then she could renounce her title and her right to the throne, in order for her to marry Peter.
Princess Margaret ended the relationship in 1955 and eventually got married to Antony Armstrong Jones.