The adult sister of a 9-year-old boy who is charged with killing their mother said he has mental health problems and probably just "snapped" on the day of the shooting.

"My mom would not want him to be labeled a murderer," Reagan Martin, 23, told MLive.com. "Her whole world revolved around him."

Police said the 51-year-old mother was found dead Monday in her home in southern Michigan's St. Joseph County. Martin said her brother was adopted in 2016 but has lived with the family since he was very young.

The Associated Press is not identifying the mother because to do so would identify the boy. Martin has a different last name.

"I am unable to comment on the facts or status of the case," said T. J. Reed, an attorney for the boy. "It is important to allow this case to properly move through the legal process and to not pre-judge this child based on partial or unknown facts."

Martin said her brother was diagnosed with a mood disorder that worsened in the last two weeks when his medication was changed. She said he was "hitting, kicking and throwing stuff."

She said she thinks he "snapped" after he was kept home from school, and that she doesn't know how he got access to a gun, as the family's hunting rifles were usually locked up.

"I don't think he meant to do it. I think (it) was probably a scare tactic," Martin said.

She said she spent time with her mother and brother Sunday.

"He was playing with my kids," Martin said. "He was happy. My mom was happy."

Sheriff Bradley Balk said the boy underwent a psychiatric evaluation at a state-run juvenile facility after his arrest.

"I'm angry; don't get me wrong," Martin said. "But he's my little brother and he needs help. I hope he gets it."