By winning their semifinal matches on Thursday here at the Australian Open, Serena and Venus guaranteed that one of them will fly home with the sterling trophy. Since they share a house in Palm Beach Gardens, Florida, it probably doesn't really matter who wins.
Another way to slice it: Serena (22 Grand Slam singles titles) and Venus (seven) have more than any other active women. This will be their 28th meeting.
The last all-Williams final came at Wimbledon in 2009, which also was Venus' last major final. Her only other Australian Open final came in 2003.
Venus, 36, was first into Saturday's final with a 6-7 (3), 6-2, 6-3 victory over fellow Californian Coco Vandeweghe -- the only semifinalist younger than 30.
Serena, 35, followed quickly, hammering the unseeded but inspirational Mirjana Lucic-Baroni 6-2, 6-3. It was over in only 50 minutes, rare for a major semifinal.
"A total inspiration," Serena said in her on-court interview of her sister. "My big sister, she's basically my world and my life. I was so happy for her, really.
"For us both to be in the final is the biggest dream come true for us."
The crowd at Rod Laver Stadium, while deeply appreciative of both efforts, understood which one was more significant. The astute patrons gave Venus a prolonged standing ovation as she ricocheted around the court like a sugared-up 6-year-old.
Venus is the oldest woman to reach an Australian Open final in the Open era and the second oldest to play a Grand Slam final, after 37-year-old Martina Navratilova at Wimbledon in 1994. Venus and Serena's combined age of 71 will be the oldest of a women's final in the Open era, with the previous record coming at the 2015 US Open final between Flavia Pennetta and Roberta Vinci (66 years).
"Everybody has their moment in the sun; maybe mine has gone for a while. I've got nothing else to do," Venus said.
A victory here by Serena -- or Roger Federer orRafael Nadal, for that matter -- would not approach the magnitude of a Venus title. It's almost a footnote now, but six years ago, Venus was diagnosed with Sjogren's syndrome, a debilitating autoimmune disease that curtailed her ability to practice and train.
On Thursday, Venus, the seven-time Grand Slam champion,was playing in her 21st major semifinal, while Vandeweghe was in her first. The oddsmakers were unimpressed and installed Vandeweghe as a significant favorite.
But serving like it was 1999, Venus survived a briefly close encounter because of one weapon. Her serve was virtually unassailable. She stroked 11 aces -- one more than Vandeweghe -- and saved 12 of 13 break points.
And when it was finally over, after she converted her third match point, Venus tossed her racket and, for a spell, seemed to lose her mind. She did a madcap 720-degree spin (repeated a little later), did a little dance and then was consumed by laughter as her accomplishment began to dawn on her.
"Oh, my gosh, it means so much, mostly because she played so well," Venus said in her on-court interview, at one point dissolving into a giddy unknown language.
"That moment was just joy," she said later. "It was a heartfelt match. If the match is 6-2, 6-2, you know, the moment is kind of clear that it's going to happen. But she played so well. There was never a moment where she wasn't just hitting the ball amazing and striking the ball with just such precision.
"It's always very satisfying to be able to get through in such a big match against an opponent who was just on fire."
The match required 2 hours, 26 minutes, making it the longest women's match of the second week here.
"I would give anything," Venus said, "to see [Serena] across the net from me on Saturday."
And that's just what happened.
The 34-year-old Lucic-Baroni, who went an incredible 17-plus years between major semifinals, was a longer shot than Venus. After beating No. 3 seedAgnieszka Radwanska in the second round and No. 5 seedKarolina Pliskovain the quarters, she seemed spent. Wearing wraps on her left thigh and calf, Lucic-Baroni was often left flat-footed by Serena's superior groundstrokes.
Lucic-Baroni took the time to take a selfie before she walked off the court and was embraced by the crowd.
Meanwhile, Serena's final flourish was more sedate than Venus'. She waved gently to the crowd.
A win over Venus would give Serena 23 Grand Slam singles titles, pushing her one ahead of Steffi Graf and setting an Open-era record.
"She's my toughest opponent," Serena said. "No one's ever beaten me as much as Venus has."
True enough. Serena holds a 16-11 head-to-head edge going back to 1998. Their last meeting resulted in a three-set victory for Serena at the 2015 US Open.
"I just feel like no matter what happens, we've won," Serena said. "She's been through a lot. I've been through a lot. I look forward to it.
"A Williams is going to win this tournament."
Serena advances to face Venus in Australian Open final
Serena Williams defeats Mirjana Lucic-Baroni in 50 minutes to win her semifinal match and set up a showdown with sister Venus Williams in the final of the Australian Open.
Venus Williams advances to Australian Open final
Venus Williams defeats Coco Vandeweghe in the semifinals of the Australian Open to advance to a Grand Slam final for the first time since 2009.
Serena looking forward to facing Venus in final
Serena Williams breaks down what it will be like facing her sister Venus in the Australian Open final.