Conor Bryan Hourigan left his victim bruised and concussed for showing affection towards his sister.
He pleaded guilty to assault with intent to injure when he appeared in the Whanganui District Court on Wednesday.
Judge Philip Crayton said Hourigan had known the victim for some time.
"As a result of the victim's affection towards your sister you became violent. You assaulted him in a manner which cannot be described as anything but a serious assault. You left the victim with serious bruising and injury."
The victim also suffered concussion and was unable to work for a period.