Why did a surgeon once give his own blood to save his sister?

Because she was bleeding to death and he had blood to give. The American surgeon William Stewart Halsted, who performed one of the first blood transfusions in the United States, was called to see his sister after she had given birth and found her dying from blood loss. In a bold move, he drew his own blood, transfused it into her, and then operated to save her life. A blood transfusion does exactly that, putting blood products into a person's circulation to replace what they've lost, but for centuries it was closer to a gamble than a treatment, because doctors believed all blood was the same.

The gamble started early. In the 1660s, physician Richard Lower performed the first reliably documented transfusion, moving blood between two dogs in front of the Royal Society. In 1667, Jean-Baptiste Denys transfused sheep's blood into a 15-year-old boy, who survived, likely because only a small amount was given; later patients of his died after repeated transfusions. In London, Lower gave sheep's blood to a man named Arthur Coga, based on the belief that a gentle lamb's blood might calm an agitated mind; Coga was paid 20 shillings to take part. The backlash to these risky animal-to-human experiments was so strong that the Royal Society, the French government, and later the Vatican all moved to ban the practice, and transfusion all but vanished for 150 years.

Human-to-human transfusion fared a little better: in 1818, obstetrician James Blundell used a woman's own husband as a donor to treat her after childbirth, and it worked, but by the late 19th century transfusion was still seen as risky and dubious, and largely shunned. The real breakthrough came in 1901, when Karl Landsteiner discovered that people have different blood groups, and that mixing incompatible ones makes the immune system attack the donor's red blood cells, sometimes fatally. His work gave transfusion a scientific basis and made it far safer, because blood groups could now be matched, and it won him the 1930 Nobel Prize.