Serena broke twice in the first set and lost just seven points on serve in her last match of the year. She looked sharper than Venus in every facet of a match that featured few long rallies and little of the spectacular tennis the two have provided in some of their previous meetings.
Serena sealed the match with a crosscourt forehand winner, and celebrated calmly with a simple fist pump before hugging her sister at the net.
"It feels great," said Serena, who also won the WTA Tour's season-ending event in 2001. "I totally didn't expect to come here and win."
It was Serena's fourth straight win over her sister. She also beat Venus in the round-robin stage of the Doha tournament, the Wimbledon final and the semifinals in Miami this year. She leads their head-to-head record 13-10.
"Playing a final against Venus is really tough," Serena said. "Even though she wasn't really feeling great, she kept hitting every ball back."
Serena earned $1.55 million for the win after finishing the event undefeated. She also clinched the year-end No. 1 ranking earlier this week after Dinara Safina of Russia pulled out injured in her first match.
The tournament featured the eight top-ranked women in the world divided into two groups, with the semifinalsts decided by a round-robin stage.
Venus lost two of her three round-robin matches but still advanced. The defending champion looked tired after rallying for a three-set win over Jelena Jankovic on Saturday, and each of her other group matches went three sets.
She played with her left knee strapped while Serena had strapping on her left thigh.
"It was the end of the season, so I have no complaints," Venus said about her fitness level. "You have to show up and play no matter what. So that had nothing to do with it."
Serena, who advanced after U.S. Open finalist Caroline Wozniacki retired injured in the semifinals, held five of her last seven service games at love and lost just one point in each of the other two.
It was her third tournament win of the season after Wimbledon and the Australian Open.
"I haven't won a tournament that wasn't a Grand Slam in a while, so that was even more exciting," she said. "My losing streak in (other) tournaments in over."
Copyright 2009 The Associated Press.