Victoria County authorities say the children's parents were at the home in Telferner, South Texas, when the shooting happened late Wednesday.
Authorities are investigating Friday.
Chief Deputy Terry Simons tells the Victoria Advocate that investigators are trying to determine why the children were left unsupervised. The parents drove their wounded son to a nearby grocery to meet with paramedics, who then rushed the boy to a hospital.
Authorities have not released the name and gender of the other sibling.
Information from: The Victoria Advocate