A 1-year-old girl and her 3-year-old brother were stabbed to death in Monday evening.
The children's 23-year-old mother was also seriously injured in the attack.
The main suspect in the case handed herself over to police in Germany at around 5pm on Thursday after an international warrant had been put out for her arrest earlier in the day.
"She stopped a police car and said she believed the police were searching for her," Thomas Klinger, a spokesman for the German prosecutor's office, told Aftonbladet.
The woman was being held for questioning in Hanover, said Klinger. She had not confessed to the crimes of which she was accused, he added.
A decision is to be taken tomorrow as to whether the suspect should be extradited to Sweden.
The suspect is a foreign national and lives in the Stockholm region. She travelled to Germany on Tuesday evening, one day after the double murder in Arboga, a family member told news agency TT.
Several websites contain pictures and information showing strong ties between the woman and the injured mother's new live-in boyfriend.
Two police technicians from Västmanland county have been examining the woman's apartment in a suburb south of Stockholm.
Investigators have now cleared the children's 28-year-old father, initially the prime suspect, of any involvement in the case.
Prosecutor Frieda Gummesson submitted the new warrant on Thursday morning. Shortly before noon police stated that they had not taken any coercive measures or carried out any home searches in the hunt for the suspect.
The children and their mother were found with serious stab wounds at their home in Arboga on Monday evening. The children later died from their injuries.
Police have not yet been able to speak to the mother, who was seriously injured and remains under observation at Uppsala University Hospital.
According to the newspaper Stockholm City, several guards are on duty outside the victim's room.
“We need to make sure that the woman is safe and undisturbed,” said police spokesperson Börje Strömberg.
He points out that there are no specific threats against the woman but that doesn't mean that the perpetrator might not make another attempt to harm her.
“Since the man who was previously head denied any involvement we were forced to look at other alternatives and it is within that context that we became interested in the current suspect,” said Strömberg.