Detective Troy Nitschke says a 20-year-old was killed and a 15-year-old was injured and called police after the shooting Thursday.
The Milwaukee Journal Sentinel reports the shooting happened inside a house in the northeast Milwaukee suburb.
A police spokesman says when when officers arrived, they found the wounded 15-year-old attending to the 20-year-old victim. Officers pulled the 20-year-old to the driveway of a neighboring home first before administering first-aid because they did not know where the shooter was.
The suspect later surrendered without incident at the house.
The younger brother was taken to Children's Hospital of Wisconsin. Officers didn't have an update on his condition Thursday afternoon.