Rory sustained the fracture playing for Glasgow in Friday's 20-8 defeat to the Borders at Firhill and will miss the remainder of the Six Nations Championship as a result.
Sean and Rory became the first brothers since Gavin and Scott Hastings to pull on the dark blue jersey of their country and both were hoping for a long run together for Scotland.
But Rory has seen Hugo Southwell oust him from the starting XV and Edinburgh utility back Simon Webster keep him off the substitutes' bench - and now injury has robbed him of the chance of putting pressure on that duo for the final two matches with Ireland and Italy.
Sean, one of the stars of Scotland's revival this season, is hopeful the lay-off will prove only temporary, however.
He said to Sporting Life: "I'm gutted for Rory because that's two injuries in a row. He did his cheekbone a week ago and thought he was going to be out but then got the OK to play.
"Now he has done his jaw, but it's just one of these things. He hasn't needed his teeth wired up, just a plate in his jaw, so it could only be four weeks rather than months and months.
"It's been nice when we've played together. You always look out for your brother, especially your younger brother, and hopefully he'll make a speedy recovery and be back in the team at some point soon."