Jenny turns around and lifts her tail, showing her equine pussy to Katlyn and saying "Don't resist Kat, give in to your desires."
From this new angle and from his point of view, it was the first time Jeff saw his sister as a centaur. Seeing Katlyn's hard equine cock, he started to 'salivate', which Jenny instinctively understood that he was into it too. Without his girlfriend noticing, a very strong feeling of hunger also hit him. With the center of his being being Jenny's rear, it was not just excitement for the pleasure of being fucked, but a need, he had to 'eat' and Katlyn's semen seemed like an extremely appetizing meal to him.
Katlyn tried to look away, but she couldn't stop thinking about it, and Jenny got closer and closer to tease her.
"OK, you win, I'm going to fuck you." Katlyn said. She then awkwardly got up and straddled Jenny. Jeff saw the tip of his sister's dick approaching his 'mouth', her balls full of 'juicy food' dangling ready to feed him. This sensation was so satisfying that Jeff felt it as his true nature, even thinking that it would be a shame if he returned to being human.
Jenny also loved the feeling of giving herself completely to the other centaur.