After Dad is killed, son takes charge as timid mom surrenders control. He eventually coats her vaginal walls with incestuous sperm and makes her his woman.
On a trip with my wife and me my mother-in-law tricked me into fucking her. On a different occasion, it was only proper to take in my sister-in-law, too.
Brad and Mary find a porn magazine that starts them exploring sexually with each other. Their mother finds out and decides to help them back to the correct path.