Officials say that the boy turned himself in at the hospital, where a family member took him there.
According to law enforcement, deputies were dispatched to a weapons disturbance at the Park at North Vista Apartments in the 300 block of North Vista Drive at 6:05 p.m.
Precinct 4 deputies said the girl was taken to the hospital after suffering stab wounds to her torso, but she is expected to be OK and was released after treatment.
Authorities said the 12-year-old boy ran away, but he was later located and taken into custody. They added that the boy was charged with aggravated assault of a family member.
Officials say a guardian was at the apartment during the incident, but details about the guardian's relationship with the children are unclear.