What does John do?

Cheap, But Practical

As Johns lust continues to build, the squeaks of his sisters new rubber body strike a moment of inspiration in him. He snaps his fingers mid-thrust and instantly finish the transformation of his sister.

In an instant, he had gone from incestuous lovemaking, to masturbating with a cheap adult toy.

He had turned her in to a cheap inflatable blow-up doll. The kind you could buy in a local sexshop for $20 Dollars or so. Her features were turned to crude and inflatable versions of their original shape.
Her body had become a simple cylinder with two round spheres for her chest, with bright pink nipple caps.
Her limbs had been reduces to simple tubes that ended with stubs.
Her face had- largely remained the same... Aside from the fact it was now just a photo printed onto the spherical head of the doll.
And finally, her 3 orificies had been turned from nice tight flesh, into slick and simple holes.