World number one and defending champion Serena came from a set down to prevail in three against 11th-seeded Dane Caroline Wozniacki in the quarter-finals, winning 4-6 7-5 7-5 in a thriller in Montreal.
Venus also overturned a first-set deficit to beat Spanish 14th seed Carla Suarez Navarro 4-6 6-2 6-3 in two hours 11 minutes - reaching her first semi since taking out the Dubai Tennis Championships in February.
The American sisters are now set for their 25th career meeting, with Serena leading the head-to-head record 14-10.
"I definitely don't like playing her - I think I've lost to her more than anyone else on the tour," Serena said after edging Wozniacki after two hours and 41 minutes.
"So it's definitely not a fun match for me, to be honest. But she's tough. She has a great serve. She runs every ball down. She has a great backhand. She hits winners off the forehand.
"She just does everything well, so it's not an ideal match-up for anyone, to be honest."
Serena, who improved to 7-1 against former world number one Wozniacki, made a slow start at the WTA Tour event, losing the first set 6-4.
She soon found herself trailing 4-2 and down 15-30 on serve in the seventh game of the second but the 17-time grand slam champion, who finished with 15 aces, made it 30-30 and that sparked a run of 10 unanswered points.
Serena won seven of the next eight games to win the second set and take a 2-0 lead in the third, and despite a stern challenge from Wozniacki, she managed to hold on for victory.
Russian Ekaterina Makarova earned a final-four clash with third seed Agnieszka Radwanska after defeating American qualifier Coco Vandeweghe.
Makarova proved too strong for Vandeweghe, closing out the match 6-1 4-6 6-1.
Next up is Poland's Radwanska, who disposed of eighth-seeded Belarusian Victoria Azarenka 6-2 6-2 in the quarters.