The Farmer's Daughters (1976). Shep and Kate live on a farm with their three daughters - Martha, Jane and Beth. Early in the film, the daughters spy on their parents having sex. When Fred, a farm hand, catches them, they rape and humiliate him.
Meanwhile, three escape convicts find the farm and a gun. They rape Kate making Shep watch. When the daughters get back home, the convicts take the entire family outside for some games including both oral and anal sex.
While the convicts are engrossed in their games, Fred arrives on the scene and manages to get ahold of the gun. Still humiliated from what the three daughters did to him, he starts his own games including making one of the daughters give Shep (her father) oral and forcing all four of the women into a group lesbian act.