The alarms for the tragic event sounded shortly before 10 at night, when someone found two children lying on the ground and badly injured in front of an apartment building in Hasselby, northwest of Stockholm.
The bodies of the two siblings had serious injuries, some of them allegedly caused before the fall.
Both victims were taken by an ambulance helicopter to hospital, where one of them died of the injuries on Monday.
The other child is being cared for life-threatening injuries, according to reports by the Swedish newspaper Aftonbladet.
The Police suspect that both children were first stabbed inside of the home and then thrown from a height of 15 meters, according to local media reports.
Inside the house, police arrested a man and a woman, who are now being investigated for murder and attempted murder, the prosecutor's office announced Monday.
According to Swedish media, the suspects are the children's parents. One of the suspects - the father - was also injured, so it is being investigated whether he injured himself after attacking the children.
Both victims are younger than 10 years old.