The guerrillas identified the killer as Marco Antonio Manyoma, who it said had left the FARC last month, "stealing guns" and other items.
In a statement, the guerrillas identified the killer as Marco Antonio Manyoma, who it said had left the FARC last month, "stealing guns" and other items.
It said Manyoma murdered his social activist sibling Emilsen Manyoma and her husband on January 14 in the village of Zaragoza in western Colombia.
The sister had reportedly warned Manyoma to stop drinking and behave himself, apparently sparking his anger, the FARC said.
The rebel group and the government are wary of violent incidents that might derail their revisited peace accord adopted in November, a month after voters rejected a first attempt at peace.
The FARC on Wednesday said its forces in central and southern Colombia battled renegade members opposed to the historic accord.
The deal aims to definitively end their half-century-old conflict.
The government has warned that those involved in fighting at this stage in the peace process will not be covered by an amnesty included in the peace deal.
Civil groups in some rural areas have reported that various of their leaders have been assassinated in recent months as the peace process takes shape.
The FARC says it has some 5,700 fighters waiting in demobilization zones to disarm over the coming months.