The sisters clinched at least a silver medal by beating Maria Kirilenko and Nadia Petrova of Russia 7-5, 6-4 in the semifinals Saturday.
"We've put ourselves in a good position," Venus said. "We'll try to go for it again tomorrow. It's amazing to do it with your sister."
Their opponents in the final will be Andrea Hlavackova and Lucie Hradecka of the Czech Republic.
"We talked for four years to be here in this position," Venus said. "It means a lot to us. The Olympics is the epic, the height of our careers."
They've played that way in doubles, winning every set through four rounds. The Williamses never faced a break point in their latest win but needed six match points, all in the final game, to close it out.
They then embraced at the net.
The sisters won the gold in doubles in 2000 and 2008. Venus also won the gold in singles in 2000.
Serena has a chance to become tennis' first double gold medallist at an Olympics since her sister. Serena beat Maria Sharapova on Saturday to win her first singles gold and complete a career Golden Slam in singles.
"I'm really just impressed that I'm doing well and playing well," Serena said, "and I'm happy and I'm energized and I'm healthy, and I have so much room for more, and I want to do more. And that makes me really, really excited."