Flirty Night In

The evening begins routinely -- the stepdad, back from a demanding day at work, parks himself in front of the gigantic flatscreen TV. The stepson joins him on the leather couch, both engrossed in the latest football game. As the night wears on, playful banter and cheering for the rivalling teams slowly evolve into flirtatious teasing and daring quips.
The game intensifies as does the atmosphere between the two -- with every touchdown, execute a playful or flirty move, culminating in a passionate final touchdown celebration.
You can bring the scenario to a boiling point, perhaps, with a friendly bet resulting in a forfeit that sets the stage for a more intimate interaction. Encourage each other's confidence and protectiveness to create an ideally satisfying shared experience.

The house needs to be private and comfortable. Adjust the room temperature to be warm and cozy. You might want to install a premium home theater system if you don't already have one for the ultimate experience. Prepare delicious snacks that you both love, and keep a bottle of high-quality whisky or any other desired beverage for added relaxation.
Set the ambiance with dimmed lights and keep comfortable cushions and blankets handy around the leather couch. Consider wearing casual but sexy loungewear to play the part.
To add suspense, prepare a couple of challenging or titillating forfeits to be used as bets during the game.

Firstly, ensure that the familial dynamic of the roleplay is purely an adult fiction and everyone is comfortable with it. Prior communication is always helpful. Consent and comfort must be the topmost priority, no matter how daring the roleplay gets.
Secondly, to double the fun and keep the spontaneity flowing, try not to plan the specific moves during the game. Let the game guide your actions and reactions leading to a much more organic experience.
Lastly, remember that being protective and cheerful is equally important in this scenario. Any 'dares' or 'penalties' should be executed with a cheerful and positive spirit. At the end of the day, have fun and enjoy each other's company.