The shooting happened Sunday evening at an apartment complex in northwest Harris County, according to the Harris County Sheriff's Office.
Investigators believe the two sisters were home in the apartment with five adults, including their mother and stepfather.
The adults believed at least one of them was watching the girls, when they were in fact unsupervised in a bedroom, according to Sheriff Ed Gonzalez
Somehow the 3-year-old got a hold 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.
Gonzalez urges people to ensure their guns are stored where kids cannot reach them, especially entering spring break week, when 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.
Officials said it is not clear at this point if anyone will face charges.