In what year did Serena Williams and her sister Venus Williams win their first Grand Slam title as a double?

Answer 1

1999
Serene was 17, Venus 18, playing at the French Open. The win followed several doubles titles for each but playing with other partners. The win was shortly before they won the doubles title at the US Open in the same year, and Serena won her first Grand Slam singles title, at the same US Open.