The McEnroe brothers were a minor force but mostly due to the elder Mac's on-court success and ability to be a virulent mouthpiece. Patrick, the younger was a field pro with diminished rankings who wisely chose to get into the broadcast booth.
In curling the legendary Watson Brothers of Manitoba (Ken & Grant) were awesome but that goes back before most of our parents were even born.
Same with the Richardsons of Saskatchewan (Ernie, Sam & 1 other)
Few great brother acts in pro hockey like the Sutters and Staals but their accomplishments pale in comparison to the Williams.
Bobby Hull and Brett Hull were incredible but they weren't brothers - they were father and son. They are the best father and son act in pro sports immo.
In ladies golf the Rangutan sisters are now the big story on campus.
But nobody comes close to the Williams sisters act. Nobody.
Serena is 36 and now has a child to tow around - she won't be as ultra-focused as she was last 20 years - BUT she's still got game, perhaps one more major title and a few tour titles to slurp up. THe elder sis, now 38 struggles more often than not- but her natural talent allows her to slither by tour fawns and advance to later rounds - where she's more often than not put down by Top 30 tour players.
How 'bout wrestling then (or rasslin' as Vince McMahon calls it)?
The Steiner brothers, Rick & Scott - terrorized rings for the better part of 3 decades with their erstwhile brand of mayhem, cruelty and pure athleticism. Nobody really liked them but they sure put on a show. Scott went to the dark side about mid-way thru their careers - started roiding up in a most serious way - and turned himself into a hulked up freak!
The Dufour-Lapointe sisters (Justine and Chloe) won the moguls' Gold and Silver during the 2014 Olympics.
The Brownlee brothers (Alistair and Jonny) won the triathlon's Gold and Silver during the 2016 Olympics.
The Klitschko brothers (Vitali and Wladimir) dominated boxing heavyweight division for years.
Also, even though technically it is a team sport, the Bryan brothers (Bob and Mike) dominated tennis doubles' for years.
I would also argue that the Murray brothers (Andy and Jamie) have been successful respectively in tennis' singles and doubles, while the Schleck brothers (Frank and Andy) were among the best climbers in professional cycling for years.
The Klitschko Bros. were 6'6" and 6'7" Russian monsters who pretty much knocked out all comers during their various reigns of terror in heavyweight pro boxing. Pretty much killed the division immo due to their dominance - or lack of anyone else who could stand up to them.
The Sedin Brothers - even with zippo Stanley Cups are the greatest brother act in hockey. The fact they played for the same pro team all their career is remarkable.
One guy a scorer, the other a play-maker. Took a ton of garbage from other players without great complaint.
Peter Stasny was amazing during his run in the WHA and NHL. The other two Stasnys were above average NHL players.
The Gretzkys were still the best brother act (Keith & Wayne) although Wayne accounted for nearly 99.9% of the scoring!
Yesss!!! Rocket & Pocket-Rocket. Very balanced careers. Rocket the older brother who was a 1-man dynamo during his near 16 year career - 544 goals and lotsa Cups.
Henri - the younger - great tenacious play-maker who could contribute goals when required.
Both guys played til each was nearly 40 - and pretty effective, too!
Best brother act in hockey, by far! (even if they didn't have the collective stats of the Gretzky brothers, lol)