Former soldier Barry Rogers and his mother Penelope John have been convicted at Swansea Crown Court of murdering his 84-year-old grandmother Betty Guy.
Rogers, 33, used a pillow to smother Mrs Guy after John had given her mother a cocktail of pills and whisky on November 7 2011.
Mrs Guy's death was initially thought to be from natural causes and no post-mortem examination was carried out.
They came under suspicion four years later when an ex-partner of Rogers told police he said he had killed his grandmother by putting a pillow over her face.
Rogers, of Fishguard and John of St Dogmaels denied murder but were convicted by a jury of five women and seven men following a trial at Swansea Crown Court.
After taking the verdict, Mr Justice Lewis told them he would sentence them to life imprisonment on Thursday.
He said: "You have been found guilty of the murder of Betty Guy.
"I order that both of you be remanded in custody.
"I direct that you be brought back to court on Thursday February 8 whereupon I will sentence you to the sentence required by the law which is life imprisonment and the appropriate minimum term."