It's deplorable to sleep with a member of your family.
Steve has a stepsister Sally who he always found attractive, and they're both out drinking together and the bar is closing.
Steve wants to continue having a good time with Sally.
Steve and Sally walk to a nearby diner and have a long, rambling and entertaining conversation over a late night meal.
Steve and Sally go home and collapse drunkenly into eachother's arms in bed.
The next morning, the two of them have a good time talking about what they did the previous night.
When they wake up next morning, they're both horrified at what they've done.