Trying something strange...

Following your father downstairs you join him outside where he had BBQ'ed you some patties. You glimpse as his erect cock, to make sure he hadn't calmed down any on the way downstairs before trying something very interesting.
With Rachel sunbathing, on her back and off to the side, you wait until your father has given you your food and began eating his own, before carefully slipping the ring back onto your rear, making your tight hole appear on your mothers forehead. You wondered how the interaction would go, more than anything - she obviously couldn't see it and even if she did react to it, it would be minimal...
But the plan worked, your father soon looked over to his wife and noticing the hole. He approached her, slipping his shorts off and she seemed totally indifferent, her eyes closed as she lay there. He straddled her shoulders and with a moan began to poke his tip against your unseasoned asshole, letting you feel his impressive size tease you for a few moments, before he slowly began to ease in, pushing until his balls slapped Rachel's face and his cock was hilted inside you.
You could hear him groan with pleasure, from across the garden, as he began to thrust, sliding in and out of your hole quickly - although they were soon overturned by your own moans, growing ever louder as he continued to fuck You.
As he sped up you heard the slapping of his fat balls against your mothers face and it only turned you on more, watching her relaxing as she was repeatedly slapped by the heavy, hairy nuts.

Quickly precum started to build up around his tip, lubing you for much smoother thrusting as you found yourself getting close to an orgasm. You couldn't imagine he was far off if you were getting close from being fucked and now your moans drew both of their attention you had to stuff your mouth full of burger to act less out of the ordinary.
With the stain growing in your shorts, as you came again, you could feel the thick, warm cum of your father being pumped inside of you, filling you to the brim in such a pleasurable way, only made funnier when he pulled out and some of his cum leaked down your mothers face, looking like he had cum all over her face.

An interesting experience, but a pleasurable one, you decide to remove the ring for now and finish your lunch. You could play more later and this burger WAS delicious, after all.