The brothers, now 23 and 25, are scheduled to appear in Lancaster County Court on July 12 for a guilty plea, records show.
Since the brothers were arrested, prosecutors have said they could face the death penalty.
An attorney for Cody Wunder could not be reached. Jeffrey Conrad, attorney for Kyle Wunder, said Wednesday the plea agreement would be a relief to his client.
"The facts, as alleged, were egregious," Conrad said. "So the threat looming over him was great. He wants to take responsibility and move on."
Kyle Wunder will plead guilty to first-degree murder, Conrad said. The maximum sentence for both brothers would be life in prison.
Harmer is scheduled to stand trial Aug. 5 on charges of criminal homicide, robbery, conspiracy, and other offenses.
The crime shocked residents of the small community about a half-hour from Lancaster.
Authorities have said Harmer schemed with the brothers to rob Herr because Harmer knew he had $200,000 cash in a safe. Harmer and Herr knew each other from work, police have said, and Herr received the money from a settlement related to the 2009 death of his wife.
Harmer allegedly waited outside Herr's home on the night of Aug. 17 as the Wunder brothers shot open a back door. Herr confronted them with a gun, shooting Kyle Wunder in a leg, police said. Cody Wunder then told his brother to shoot Herr, the police report said.
Harmer's girlfriend later told police she heard Kyle Wunder admit to the crime by saying, "Of course I'm going to shoot the guy. He shot my brother."
Contact Allison Steele at 610-313-8113 or firstname.lastname@example.org.