While he was restrained by others at the party, Rapana and Rio Davis continued their confrontation at the back of the property, during which the defendant pulled out a hunting knife and inflicted multiple stab wounds.
When those inside heard about what had happened, they released Chevy Davis who confronted Rapana.
He was fatally stabbed.
Rapana and his wife left the scene and hid the murder weapon on the way to a friend's house.
They changed their clothes and washed the garments they had been wearing before returning home, where police found them.
"You stated during the course of your police interview that you had acted in self-defence because you were scared for your life and that of your wife," Justice Sarah Katz said.
Defence lawyer Karl Trotter told the court his client should avoid the mandatory life sentence because it would be "manifestly unjust" but the judge disagreed.
Justice Katz accepted there was a degree of provocation and elements of self-defence but emphasised Rapana was the only one to use a weapon during the fracas.
Chevy Davis's father Robert said grief was his "constant companion" and Rio Davis said he would never get over what happened that night.
"He has lasting physical scars and injuries from your attack on him, and he also deeply misses his cousin Chevy, who he was close to," the judge said.
"He cannot understand why someone would bring a knife to a family birthday party."
Justice Katz accepted Rapana was genuinely remorseful and discussed the loss both families would face.
"As a result of your brutal and impulsive actions that night the Davis family have lost a beloved family member. Your own family will lose the presence of a husband and father in their lives for many years to come. You are no doubt deeply aware of that reality. You will have to live with the consequences of your impulsive actions that night for the rest of your life," she said.
Rapana, whose only previous conviction came as a teenager, was sentenced to life imprisonment with a minimum non-parole period of 10 and a half years.