There was something Joanie had never known about Sue. Sue had loved and desired Joanie since puberty. Being reared in a strict religious household made her afraid to act on any sexual feelings, let alone those for her female cousin. So she had try to be like Joanie as much as possible, even emulating her devotion to dogs, and, in utter secrecy, fantasizing about Joanie while masturbating. Sitting at the Chronivac, Sue had realized that as much as she loved the idea of being Joanie/Marlene's owner, there was an even better role available.
Marlene's mate.
Sue had set up a time delay on the Chronivac to put Marlene into heat five minutes after her change into Sam. Sam was a powerful, dominant alpha male dog (quite a change from the shy, withdrawn Sue) who would be able to overcome rivals for access to the female. What was left of Sue in Sam revelled in the powerful, utterly shameless sexual feelings of a male canine. He wished that Sue's parents, who had taught her to be ashamed of her sexuality, were there, and they could see their daughter-turned-dog mounting her cousin unashamed before the world.
Marlene had seen Sue's transformation, and admired the powerful body and clean outline of the new dog. As a bitch, she realized that the new dog would father a fine litter of puppies. As Sam and Marlene joined together, doing what their magnificent new bodies were designed to do, both realized that they never wanted to go back to being human.