As he drank the milk from his sister's left breast, Jeff could feel his body changing. Nevertheless, he was addicted to her milk, and resigned himself to whatever changes the milk may cause. Jeff rubbed his cock through his pants as his body transformed.
A warm feeling covered Jeff's skin; he paid no mind to it, still latched onto his sister's breast. The muscles in his arm, which he once enhanced using the Chronivac, were now less defined. The pecs which one stuck out through Jeff's shirt became soft, and his chiselled abs lost their definition. Jeff understood what was happening now, and he welcomed the change. His sister's warm milk was causing him to gain weight, and fast.
Jeff's arms became flabby, and his torso became a blob, losing the V-taper he once had. The fabric of his pants began to tear, and after a moment, they split entirely, letting Jeff's new fat ass spill forth.
Jeff's new gut spilled forward from his stomach, covering his shrinking penis. His once hard pecs became soft man boobs, reminiscent of his sister's new MILF form. His new tits rested on his engorged belly as he continued to fill his stomach with milk. With a loud tear, Jeff's shirt became tatters, and his blubber filled torso was exposed.
'You're growing a lot today, Jeff,' Natalie said, enjoying her brother's new form.
His chin became two- no- three new chins, as his cheeks swelled, changing his appearance drastically. He was once an average kid, but now, he was was the size of a king-sized bed, with no signs of stopping. Jeff continued filling himself with his sister's sweet milk, making her moan with delight. This was Natalie's favorite part of the day.
Jeff's penis, which was once an incredible 10 inches, had shrunk drastically, now measuring only 3 inches in length. Jeff wrapped his fat sausage fingers around his small cock, which was now exposed due to his pants giving way to his massive body.
Jeff, still sucking his sister's breast, looked over at his father, who was experiencing a similar fate. His once muscular body was now covered in folds, and his previously enviable cock had shrunk to a shadow of its former self. Jeff was getting horny looking at his father's new form, hoping to experience it more closely later.
Suddenly, Jeff reached an unexpected climax as his tiny cock shot cum onto his sister's legs.
'Finished, Jeff?' Natalie giggled, stroking his hair. Jeff detached himself from his sister's nipple and fell backwards onto his enormous ass. His fat body lay spread across the floor, and he questioned how he was now going to get up. He had gone from a regular 170 pounds, to now crushing the scales at over 400 pounds. Despite this, Jeff was loving his new, pudgy self, that his sister had given him.