Authorities believe the young girl was shot unintentionally by her three-year-old sister.
At about 7:30 p.m., HCSO deputies received a call about a juvenile hurt at an apartment complex at 9955 Bammel North Houston.
According to Sheriff Ed Gonzalez, investigators believe the two sisters were in the apartment with five adults, including their mother and stepfather.
Investigators said that the adults believed that at least one of them was watching the girls, who were unsupervised in the apartment bedroom.
That's when authorities believe the 3-year-old got ahold of a loaded semi-automatic pistol and accidentally shot her 4-year-old sister.
The child was pronounced dead at the scene.
"It just seems like another tragic story of a child gaining access to a firearm and hurting someone else. This time it was a fatal shot, appears to be to her sibling," Gonzalez said.
It is not clear at this point if anyone will face charges.
Gonzalez urges people to ensure their guns are stored where kids cannot reach them, especially entering Spring Break week, when many more kids will be at home.
"You've got to make sure you're a responsible gun owner. Secure your weapons in a safe place. It's got to be more than just to tell the kids not to touch the weapons," Gonzalez said.