The court in the southern town of Ellwangen convicted the man on six counts of murder and two of attempted murder, after hearing how another aunt and uncle survived the bloodbath with serious injuries.
The 27-year-old defendant is to serve some of his sentence in a psychiatric hospital.
Prosecutors and co-plaintiffs had called for the full 15-year life sentence for the crime.
The public prosecutor’s office spoke of the man’s “limited ability to control” his actions, whereas the co-plaintiffs considered him fully culpable.
The defence pushed for a 13-year custodial sentence and accommodation in a psychiatric hospital.
The shooting took place on Jan. 24 in the town of Rot am See in the south-western state of Baden-Wuerttemberg.
The defendant is said to have shot 30 bullets, killing his family members with headshots in the man’s father’s apartment block, in which he also lived.
The family had met for a funeral there.
During the trial the man said he had been planning the attack for some time, having joined a marksmen’s club and legally obtained a gun prior to the crime.
He said he had acted out of hatred towards his mother, whom he claimed had tried to poison him with female hormones.
“I’m sorry. I regret what I did,” the man told the court before his verdict was read out. (dpa/NAN)