'Summer House' West Wilson's Cousin Charged with Murdering Their Step-Grandmother

West Wilson's cousin has been arrested and charged with allegedly murdering their grandmother ... TMZ has learned.

Dakota Sweeney of Carrollton, Missouri was arrested on April 22 -- the night before West filmed the "Summer House" reunion. Carroll County law enforcement officials responded to reports of a shooting at a residence and found 75-year-old Gayle R. Wilson dead.

According to the probable cause statement, cops say a witness told them Sweeney was quietly sitting in the same room as Wilson for 30 minutes before he allegedly shot her with a handgun.

Officers also state this witness told them Sweeney and Wilson argued earlier in the evening over Sweeney not assisting with chores around the house. After taking Sweeney into custody, officers say they located a holster on his person.

Sweeney was arrested without incident at the scene, and he's since been charged with first-degree murder.

Wilson's dad, Bruce, and Sweeney's mom, Kelly, are both Gayle's stepchildren.