World number one Serena outlasted Caroline Wozniacki to clinch a 4-6 7-5 7-5 victory while Venus also came from a set down to beat Spanish 14th seed Carla Suarez Navarro, the third round conqueror of Maria Sharapova, 4-6 6-2 6-3.
Serena will take a 14-10 career lead over her older sister into the semifinal.
The 32-year-old has won their last five meetings while Venus's most recent win was in Dubai in 2009.
Serena, who lifted her third Canadian crown last year when the tournament was held in Toronto, battled back from a set and a break down to claim victory over Wozniacki in two hours and 41 minutes.
"You go in there thinking, this is not going to be routine. You have to be ready."
Wozniacki, a former world number one who is seeded 11th, claimed her only victory over the US star in the quarter-finals at Miami in 2012.
Wozniacki had been riding an eight-match winning streak, including a title in Istanbul in July.
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