Ben, the terrier scrumhalf, and elder brother Tom, a centre-turned hooker, are seeking to become the first brothers to play together in a Lions test since Gavin and Scott Hastings, also against the Wallabies, in 1989.
They have only just got over the emotion of playing together for England for the first time in November, watched by their proud father and former England scrumhalf Nick while the good times keep on rolling.
Victory over Harlequins on Saturday took Leicester, who have six players in the Lions party, into the Premiership final against Northampton, while Tom's remarkable progress last week earned him the Premiership player of the year award.
It was hardly surprising, then, that the pair said they had barely had time to think about their Lions selection, let alone the prospect of lining up together in the test side against Australia next month.
"It's surreal, trying on your kit and looking around the room at all these players you've looked up to," said Ben, who will fight it out with Welshman Mike Phillips and Ireland's Conor Murray for the scrumhalf berth.
"It's great, it's the pinnacle, but we still have a job with Leicester and we have to make sure we don't get distracted."
Ben, who six months ago was still an uncapped hopeful on the edge of England's squad, was similarly overwhelmed by the Lions' first brief get-together among the marble busts of Syon Park House in west London on Monday.
"There's an enormous amount of pride, it's something very special," he said of the feeling of sitting alongside his brother in a Lions shirt. "We'll get a little feel for it today, get to know a few people and introduce ourselves, but it's probably not until we meet up on the Sunday after the Premiership final that we'll get really stuck into it.
That Twickenham showdown on May 25 brings Tom head to head, literally, with Dylan Hartley, one of his two rivals for the Lions shirt and with whom he has been vying for the England starting berth. Wales's Richard Hibbard is the other hooker in the Lions' party.
"With Dylan it will be great," he said, "There's already a great competition between us and I'm sure that will continue for a few years - we'll be knocking into each other."
It was only four years ago that Tom, despite winning age group caps for England as a back, was persuaded by former Leicester coach Heyneke Meyer to have a go at transforming himself into a hooker
It was a highly unusual switch in professional rugby but he adapted superbly, winning the player-of-the-year award when on loan at Nottingham.
He returned to Leicester and, under the guidance of former England hooker and now club head coach Richard Cockerill, continued his rapid rise.
Tom Youngs made his England debut against Fiji in November, when Ben came off the bench to join him, and the two brothers started together against South Africa.
While Ben has continued to vie with Danny Care for the scrumhalf jersey, Tom edged ahead of Hartley to become Stuart Lancaster's first choice number two in the Six Nations.
"I haven't had time to pinch myself," Tom said after his whirlwind six months. "It's been a good season and I've enjoyed it but I've had a lot of help since changing position so I owe a lot to a lot of people.
"I enjoyed the switch and have never doubted the decision. I enjoyed it at Nottingham and back at Leicester and eventually I got here - but I can't really believe it."
The Youngs are the 14th set of brothers to tour with the Lions but only a handful have featured in the same test.
Scotland's Hastings brothers played together twice against Australia in the victorious tour of 1989 but, while both men were in the 1993 squad, only captain Gavin played in the tests.
More recently England's Rory Underwood played in all six tests of 1989 and 1993 but brother Tony's only test cap came in 1997 while Ireland's Richard, Paul and David Wallace all played Lions tests in 1993, 1997, 2001 and 2009, but never together.
The Youngs share the same favourite Lions memory, from the 1997 series in South Africa, the last time the tourists managed a series win.
"We used to watch the video over and over again," said Tom. "Scott Gibbs running over the top of Os du Randt - just brilliant."