Two Indonesian brothers have been arrested for allegedly murdering their 16-year-old sister over accusations of extramarital sex, police said Tuesday, a killing for which they could face execution if convicted.

The men, aged 20 and 30, admitted that they attacked their sister with a log and a machete because she had sex outside of marriage, authorities said of the killing on Saturday in Sulawesi island's Bantaeng regency.

The teen's family also briefly held hostage a distant relative suspected of having relations with the girl after he refused to marry her. "The suspects said they were outraged and ashamed when their sister admitted she had sex with a man who is their distant relative," local police chief Wawan Sumantri told AFP.

Police detained seven other members of the family for questioning, and to protect them from outraged local residents, he added. The brothers could face the death penalty if convicted of premeditated murder.

So-called "honour killings" -- usually involving a woman being murdered by a male relative over claims she disgraced her family -- are prevalent in some parts of South Asia but rare in Indonesia.

The teen's killing may have been linked to local norms focused on protecting family dignity, said Amnesty International Indonesia's executive director Usman Hamid. "But it's an abuse and misrepresentation of that culture," he added.