Auburn police say the kids were inside their home, located in the 4600 block of Auburn Way South, when the brother grabbed a .22 caliber rifle from another brother's closet.
At roughly 9:30 p.m., police arrived at the house and found the girl with a gunshot wound to the leg.
"He was playing with it, it was a loaded rifle and it went off," said Auburn Police Cmdr. Mike Hirman
Medics took her to Harborview Medical Center in Seattle for treatment, and police expect her to make a full recovery.
Police are now trying to figure out if the children were home alone at the time. They have yet to file charges in the case, but the investigation is ongoing.