As of my knowledge cutoff in September 2021, Serena Williams has won more Grand Slam titles than her sister Venus Williams. Serena has amassed a total of 23 Grand Slam singles titles, placing her as one of the most successful female tennis players in history. On the other hand, Venus has won 7 Grand Slam singles titles. Both sisters have made significant contributions to the sport and have had highly successful careers, but Serena holds the edge in terms of Grand Slam titles.