MELBOURNE - The Spinks boxing brothers and the Polgar chess sisters. The Charlton football brothers and Maleeva tennis sisters. Mothers and sons at the same Olympics. Twins in the same boat. Jackie Robinson broke the colour barrier in baseball while his brother Mack won silver behind Jesse Owens at the 1936 Olympics.
Families in sport fascinate us but when it comes to dazzling DNA nothing compares with them. Nothing so athletic, so game-changing, so constant, so enduring. Nothing like The Williams Sisters.
What happened to you since 1997? Maybe you moved to a new city, forgot Pete Sampras, fell in love, had a child, turned grey, fought illness, but every time you put on the TV, there they were.
Venus, Serena, the sisters. Playing, grunting, trying. Tough, resilient, proud. Changing the look, colour, style of tennis with bags, beads and bewitching tennis. Fighting rivals, racism and sexism. Impressing us, annoying us, changing us.
From one century to the next.
Together they are 71 years old, which is three years more than Karolina Pliskova, 24, Madison Keys, 21, and Garbine Muguruza, 23, put together. Together they - No. 2 and No. 17 - have 120 singles titles which is 37 more than the rest of the top 10 combined. Together they've been to 44 Grand Slam singles finals which is two more than Justine Henin (12), Martina Hingis (12), Kim Clijsters (8), Lindsay Davenport (7) and Amelie Mauresmo (3) added up.
They're two people, of course, not joined at the hip. Couldn't happen anyway. Venus is 10cm taller. One slender, one young; one elegant, one fierce; one initially better, one finally superior. In their first Grand Slam meeting, Venus wins and says: "After the match, I told her, 'Serena, I'm sorry I took you out. I didn't want to, but I had to do it'."
Their father is called an unconventional teacher. Another phrase for genius. "I planned their career before they were born," he told CBC Radio once. What planning, what careers: 30 Grand Slam singles titles as of today, nine Olympic medals, 14 Grand Slam doubles titles.
Sisters are ready-made training partners. Think of it as sporting home schooling. It is learning about defeat and having dinner with the victor. They are champions and also architects: Venus makes Serena, Serena makes Venus. As Greg Chappell, 68, the great cricketing younger brother of the great Ian Chappell, 73, says: "You learn how to hang onto a match. You learn you're not going to win if you give up. We played some real matches out there in our driveway where we were making decisions in real time. And the best players are decision makers."
The Chappells played in a team as did the Nevilles. Playing together is a joy, confronting each other in a one-on-one sport is a test. Especially when the world is gazing, writing, stoking. Wondering if you fix matches, go easy, rotate wins. It's downright disrespectful stuff to determined, dogged dames.
But we're intrigued, we want to know what it feels like, because we've fought our brothers downstairs for the football but this is scrapping with a sister for the grandest trophies. How do you retain friendship? How do you return to being silly and normal after sport divides you in public into victor and vanquished? How do you manage it before finals? By just not talking?
"No, we definitely talk," said Serena. "I think now more so than anything. Nothing can break our family. If anything, this will definitely bring us closer together, knowing that I want to see her do the best that she can possibly do. I know that she definitely wants to see me do the best that I can do."
We can rewind all the tapes and flick through 20 years of interviews, but did you ever hear anything even faintly ugly in their voices about each other? Ever hear a tone of envy, ever hear complaint or sense friction, ever hear anything less than support, ever hear anything but love?
Applaud Venus for this. Hail her for being the first Williams to emerge and face all the intense, mad scrutiny. Laud her for casting a shadow so wide in which Serena could unobtrusively develop. Praise her for the grace she displayed when overtaken by Serena, sitting forever in the players' box and cheering her on. To this duel she brought dignity.
Their matches have rarely had the raw tension and stylistic contrast and lefty-righty difference of Rafa-Roger, but effort has never been in doubt. Competitiveness swims in their capillaries. Vanity in their veins. Of course they want to win.
"When I'm playing on the court with her, I think I'm playing, like, the best competitor in the game," says Venus. "I don't think I'm chump change either, you know. I can compete against any odds. No matter what, I get out there and I compete.
"So it's like two players who really, really can compete, then also they can play tennis. Then, OK, won't be an easy match. You have to control yourself, then you also have to hopefully put your opponent in a box. This opponent is your sister, and she's super awesome. It's wonderful."
Today they return to the past. Today they meet again in a city where they first collided in 1998 in the second round. Today it will not be The Serena Show as tennis has become but The Williams Sisters as it was.
Today, Serena will be the practical favourite and Venus the sentimental one. One will win and the other will find a smile. They will talk and then fly home. Two women whose sisterhood is their enduring triumph.