Rutledge, 16, testified that he was with Graham on July 11 when she led authorities to Johnson's body in Glacier National Park. Rutledge said she told him to tell others that park rangers had found the body.
Graham, 22, cried quietly during her brother's testimony.
After Graham discovered the body, investigators questioned her further and were told she and Johnson had argued in the park over their 8-day-old marriage and that he accidentally fell from a cliff when she tried to remove his hand from her arm.
Graham has pleaded not guilty to first-degree murder, second-degree murder and making a false statement to authorities.
There were no witnesses to what happened the night of July 7.
Prosecutors said they will prove that Graham intentionally pushed Johnson from the ledge through the testimony of friends and relatives, and text messages sent by Graham before and after Johnson's death.
Andy Nelson, Graham's federal public defender, said Johnson's death was a tragic accident that occurred after she had finally worked up the nerve to talk to Johnson about her grave doubts about their marriage. When he grabbed her during a heated argument, she pushed him to remove his hand, the lawyer said.
Graham did not want to report the accident because she was afraid, the lawyer said.
The couple was married on June 29, about two years after they started dating.
But almost immediately, Graham started having "the wedding blues," U.S. Attorney Kris McLean said during opening statements.
Prosecutors presented jurors with dozens of text messages between Graham and her friend from church, Kimberly Martinez, that documented how Graham's nervous excitement at the prospect of the wedding turned into despair over the week that followed.
Graham wrote in one text the day after the wedding that she wished somebody would have asked her if the marriage was what she wanted, and that all she wanted was to be herself again.