Official White House Photo by Chuck Kennedy
"First Lady Michelle Obama waits with Caroline Kennedy Schlossberg in the Green Room of the White House before making remarks to the White House Historical Association, Oct. 31, 2011. The White House hosted a reception to honor the group's 50th anniversary."
Update: As several have observed in the comments, Caroline Kennedy wears an Alexander McQueen Slashed Sleeve Military Knit Pencil Dress, available here. The First Lady wears a two-tone pink dress by Jason Wu, which appears to be the same dress that Mrs. O wore on the cover of Vogue in March 2009, here.