Richard Zikmanis on Monday pleaded no contest to involuntary manslaughter in the March 27, 2010 shooting death of his sister Denise.
He was supposed to stand trial Monday for first degree murder but is pleading to a fourth degree felony. Zikmanis' sister, who was visiting from Montana, was shot in the head and died two days later.
Investigators said the shooting appeared to have happened after a misunderstanding between the siblings inside a Las Cruces home.
His defense is asking for probation.
Prosecutors say the case was very old and over-charged.
Prosecutor Jacinto Palomino told ABC-7 they lacked witness testimony as another reason to offer a reduced charge.
Prosecutors will ask for 18 months in prison once sentencing takes place following a 60-day diagnostic evaluation.
Zikmanis was free on bail but was finger printed and taken into custody.