I will say on May 28, 1996, when Cal Ripken hit a two-run home run in the fourth inning, giving the Orioles the lead for good with a grand slam in the seventh inning, and added another two-run home run in the ninth inning.
His brother Billy Ripken also hit a home run in the ninth inning, and the Orioles defeated the Seattle Mariners, 12-8.
It was just the second time the Ripken brothers had both hit a home run in the same game.

I don't usually like to answer my own questions, but since I have found out some more recent occurrences than the Cal & Billy Ripken case in 1996 I may as well share them in case anyone is interested.

Bret and Aaron Boone homered in the same game twice (Sept 1st 1999 and May 11th 2000), although bizarrely both times they were on opposite sides when Bret came back to Cincinatti on a road trip with the Braves.

More recent though is the case of Jason and Jeremy Giambi, who did this 4 times while team-mates at Oakland... the most recent case was August 11th 2001 in a victory over the Yankees. So as far as I know this is the most recent instance, although there are a handful of sibling duos I have yet to check out that may steal the honour.

edit - July 31st 2005, Bengie and Jose Molina both homered for the Angels in a loss at Yankee Stadium.