Serena Williams teamed with big sister Venus to win the women's doubles title at the Olympics on Sunday, adding to the gold she won a day earlier in singles.The American sisters beat Andrea Hlavackova and Lucie Hradecka of the Czech Republic 6-4, 6-4 under the roof on a rainy afternoon at the All England Club. Venus closed out the match with a backhand volley winner after the Czechs saved a pair of match points.On Saturday, Serena beat Maria Sharapova 6-0, 6-1 for the singles gold. She joined Steffi Graf as the only women to complete the Golden Slam — winning the Olympics and the four majors.Serena became tennis' first double gold medalist at an Olympics since Venus won singles and doubles at the 2000 Sydney Games. The sisters also won the doubles gold at the 2008 Beijing Olympics.The Williams sisters didn't drop a set through their five Olympic matches at Wimbledon.