(Commissioned by Beelzeboobs.)
While Johnny was "working" for Dexter to help out with the Slime Monster and around the lab in general, he let slip that his sisters are also super genius scientists that turned their Mom into a Futa. Dexter was intrigued by the thought of other geniuses working on the same tech as him and a little wary of competition, so he had Johnny bring his sisters over.
Susan and Mary agreed to try out Dexter's gender-bend ray if he'd let them experiment on his Mom. Dexter's machine proceeds to readjust the insides of the Test "sisters" silly.
(Full version can be seen here, here, Hentaifoundry, and Pixiv.)
(An alternative version of the gals with cute short hair can be seen here and here.)
SUBJECT: Dexter's Laboratory, Susan Test, Mary Test, Johnny Test