Ali Shawkat al-Sahili accused his sister of staining the family name before he lured her out to orchards near their home and shot her.
So-called "honour killings" are not unusual in certain regions but in the last four weeks there have been an unusually high number of cases reported from as far away as the UK to the Middle East.
Al-Sahili, 24, told investigators and reporters that he lured his sister Fatima, 27, to orchards near their village in Lebanon's Eastern Bekaa Valley where he shot her twice, after she "stained the family name."
Al-Sahili, who did not elaborate on how he believed Fatima had dishonoured the family, said he stuffed her body into the boot of their father's car and drove to a nearby well where he dumped her body.
So-called "honour killings" – the murder of female relatives in cases of suspected adultery, fornication or rape – are relatively rare in Lebanon, despite laws which allow courts to give reduced sentences to men convicted of them.
In September, three brothers in the Jordanian capital of Amman, admitted to hacking their two sisters to death, using axes to "cleanse the family honour".
Their admission came a day after parliament rejected an amendment that stiffens sentences for people convicted of such killings. Jordan's current penal code considers killing female relatives caught committing adultery as an act of self-defence, making sentences more lenient.
The House of Representatives rejected an amendment passed by the upper house or Senate that would toughen penalties for "honour killings" and sent it to the Senate's legal committee for revision.
Activists said the Jordanian parliament's rejection of the new law gave men "a licence to kill" their female relatives.
In early October, a British court sentenced a Muslim businessman to life imprisonment, for murdering his teenage daughter's Christian boyfriend. That was the second case in the UK of "honour killings" in just over a week.
In the first case, prosecutors told the court Mushtaq Ahmad, 40, felt 22-year old Albanian Rexhap Hassani, had damaged the family's honour by his relationship with Ahmad's daughter.
In the second reported murder, Abd Allah Yunis, an Iraqi Kurd, was also jailed for life after he repeatedly stabbed and slit the throat of his 16-year old daughter Heshu because she was dating a Christian boyfriend.
Scotland Yard police headquarters has now set up a special squad to investigate such murders.
Human rights group, Amnesty International (AI) has said "all reports of honour killings should be investigated and prosecuted."
In a report detailing its study on such cases in Pakistan, AI said "the isolation and fear of women living under the threat of honour crimes are compounded by state indifference to and complicity in womens' oppression."
Police, AI continued, almost invariably take the mans' side in honour killings or domestic murders, and rarely prosecute the killers.