A 7-year-old Colorado boy who found a shotgun in his home accidentally opened fire on his 3-year-old sister, killing her, according to police, who arrested the kids’ mom in connection with the tragedy.

Michaela Dawn Harman, 25, was busted on a felony charge of child abuse resulting in death after the early Tuesday shooting in suburban Westminster outside of Denver, police said.

“Tragic examples like this remind us of the importance to separate guns and ammunition kept in the home and to secure firearms in a locked safe or utilize other childproof safety devices,” department officials said in a statement. “Parents have a responsibility to keep children from being able to access firearms, even inside the home.”

Cops and fire officials rushed the girl — who was not immediately identified — to a hospital after responding to the family’s home, but she did not survive her “significant” injuries, police said.

Harman and another young sibling were also at the home when the shooting happened. That child and the 7-year-old boy are now in the care of Jefferson County Human Services, police said.

Harman, who is a single parent, is the owner of the shotgun, the Denver Post reports.

She remained in custody early Wednesday at Jefferson County Jail, online records show.