The guy just fell for his stepmother's melons. You can't help but feel them. And the threesome was the last straw that broke his will.
I've had sex like this several times, it's unforgettable
I don't even think I had to get him to do it. The lady appears to be quite used to anal and enjoys it to the fullest!
Somebody's jerking off to a man's ass.
It's an interesting fantasy to paint such a fat ass green! Ladies with such an ass are especially good in the crawl position, a boner is assured. Except you have to have a decent length tool. If you don't have one, I recommend laying the lady on her back and fucking her with your hands on her thighs pressed up against her belly. Otherwise you can't get the dick in deep.
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Well, half-brothers and half-sisters are not related at all, so it can't be considered something bad or immoral. It's no surprise that an adult guy and girl, with no regular sexual partners and almost daily being around each other, suddenly felt attracted on a sexual level to each other. Considering that the girl liked it (the guy then no question), I think they will continue to practice this kind of thing from time to time.