Sias will play at flyhalf for the Perth-based Force and Robert at inside center for the Bloemfontein-based Cheetahs who are hoping to end their four-match tour to Australia and New Zealand with a third straight win.
After losing 45-3 to defending champions the Chiefs, the Cheetahs beat the Highlanders 36-19 and the New South Wales Waratahs 27-26 in their last two matches. They are third in the South African conference behind the Pretoria-based Bulls and Durban-based Sharks and ahead of the Cape Town-based Stormers and Port Elizabeth-based Southern Kings.
Sias Ebersohn left the Cheetahs after a disappointing 2012 season during which he was dropped for the young Springboks star Johan Goosen. Any possibility of a showdown between Ebersohn and Goosen was ruled out this week when Goosen suffered a serious knee injury in training which could sideline him for the remainder of the Super Rugby season. His place in the Cheetahs lineup will be taken by Riaan Smith.
Ebersohn has been a key figure for the Force this season, kicking 14 points in their 19-12 win over the Queensland Reds last weekend.
Robert Ebersohn has also made his mark, scoring a try after only 47 seconds in the Cheetahs' fifth-round win over the Waratahs. The brothers have never played against each other in a competitive match and their meeting on Saturday adds spice to an already interesting match.
If the Cheetahs can bank another away win - bucking a trend in Super Rugby - they will be strongly placed to improve their standing when they return to South Africa. They face the Melbourne Rebels and Bulls at home in the next two rounds before traveling to face the Stormers and Sharks in an exacting run through the mid-season.
They have a good chance of taking their third win in five games from the match against the Force, though the Perth-based side - after starting the season with three straight losses - showed a confounding glimpse of form with last weekend's win over the Reds, champions two seasons ago.
The battle between the Ebersohn brothers adds an additional factor to a match which is already important to both teams. Sias Ebersohn is determined to prove to the Cheetahs that they erred in allowing him to leave last season.
"It's a high-profile game and the first I will ever play against Robert, apart from trial and training matches," Sias Ebersohn said. "But I am not going to concentrate the whole time on trying to show them they need me.
"I am over the Cheetahs, I am now at the Force and I have moved on. I will be concentrating on our performance as a team."
Ebersohn said he and his brother knew each other's games well, not especially because they are twins.
"I don't think it's a supernatural thing," he said. "It's more like two people who have spent their entire lives together.
"We shared the same room until age 22. I am living his emotions with him and it's been that way our whole life."
The Cheetahs will have to overcome the odds to win again on Saturday, away from their Bloemfontein base. In the history of Super Rugby, home teams have won around 61 percent of matches. This season the trend is slightly less with 15 home wins and 12 matches won by away teams.
The Chiefs split their recent matches in South Africa, losing to the Stormers before beating the Southern Kings to remain at the top of the New Zealand conference and in second place behind the ACT Brumbies overall. They return home on Friday to face the Dunedin-based Highlanders, now desperate to win after losing their first three matches.
All Blacks center Richard Kahui will make his first appearance of the season for the Chiefs after being named in the reserves for Friday's match at Hamilton. Kahui, along with scrumhalf Brendon Leonard and fullback Robbie Robinson, has overcome an injury to rejoin the Chiefs' strong roster of fit players.
But captain Craig Clarke will miss the match as a precaution after suffering a slight head knock against the Kings.
The Chiefs beat the Highlanders 41-27 in their first match this season and coach Dave Rennie said he expected a tougher test in the return match.
"We are expecting a similar battle to our first game against the Highlanders. It was physical and fast and this week will be nothing less," Rennie said.
"It will be a special game as we will be honouring Liam Messam for becoming the first person to have played 100 matches for the Chiefs. To acknowledge his impressive achievement in front of our home crowd will be a great occasion."
The Brumbies will attempt to stretch their four-match winning start to the season and seven-match winning streak away from home when they take on the Stormers at Cape Town on Saturday.
Springboks center Juan de Jongh will play his first match of the season for the Stormers after suffering a serious hamstring injury in preseason training.
"The Brumbies are the pacesetters in the 2013 Super Rugby tournament," Stormers coach Allister Coetzee said. "It will be a tough game against a quality side.
"Their set-piece is good, they put the opposition under pressure with ball in hand and it will be a good challenge for our defensive system and defense line.
"Technically, the Brumbies have been good, as they showed against the Sharks, but people tend to be overlooking the fact that this Brumbies side is a physical unit too."
All Blacks winger Zac Guildford has been named on the bench by the Christchurch-based Crusaders for their match on Saturday against the Kings. Guildford has recently ended a period of treatment for alcoholism and has been cleared to return after a disciplinary process managed by the New Zealand Rugby Union.
The Kings are under investigation for possibly using more than the permitted two foreign players during last weekend's 35-24 loss to the Chiefs. New Zealander Hadleigh Parkes started at center in the match while Argentines Tomas Leonardi and Nicolas Vergallo came off the bench.
In other sixth-round matches, the Reds will play the Bulls at Brisbane -both teams coming off a loss - the Sharks will host the Rebels, and the Waratahs will play the Auckland-based Blues.