Venus Williams upset little sister Serena 6-7, 6-2, 6-3 to register the first win over her top-ranked sibling in five years and move into the Rogers Cup final in Montreal.
The meeting marked the 25th time the sisters had squared off but the first time Venus had come out on top since a 2009 semi-finals encounter in Dubai.
World number one Serena holds a 14-11 lead in their head-to-heads after coming out on top in their previous five encounters, including the 2009 Wimbledon final.
Clashes between the pair, which at one time were an almost regular and welcome occurrence on tennis courts around the world, have become rare and their meeting in Montreal was just the second since the 2009 Tour championships.
The siblings have won 106 WTA Tour singles titles, including 24 grand slams, between them.
"It was a tough match out there," said Venus, who will meet Poland's Agnieszka Radwanska in Sunday's final. Third seed Radwanska beat Ekaterina Makarova of Russia 7-6(1) 7-6(3) to advance to her second final of the year.
"I don't think we played for a number of years, as well. It's not like we've been playing year in and year out," added Venus.
"I definitely expected a tough match. She played well. She hit so many aces.
"I think we both know when we walk out there, it's not like you're guaranteed a win. I think that's what makes it challenging for both of us."