Set sister against sister

You can see in the sisters' eyes how horny they are. It's a common side effect of youthening. Like going through puberty coming from the other direction and at 500 times the speed. The flood of renewed energy, health and glandular activity, especially when it hits a person used to an 80-year-old libido, tends to leave most women randier than a cat in heat. You could seduce them both easily in this state. You've played that game before. But today you're feeling a bit more...evil.

"Your gratitude is touching," you tell them. "But before you thank me, I should tell you I can only keep one of you this way. One of you will go back to your real age in two hours. I just am having the hardest time knowing which one of you I should keep young, though, so I youthened you both temporarily to see if that made the decision easier. But you're both so lovely now! Do either of you think you can convince me that I should choose you instead of your sister?"

Patsy gasped.

"You actually expect us to compete with each other for..."

"Look at these!" Peggy declared.

While her sister was objecting, the younger blonde had struggled out of her dress and was shaking her newly firm, round titties and pointing at them.

"Why you disloyal little trollop!" Patsy exclaimed, her green eyes flashing. "You ALWAYS did this, since the day you hit puberty! You make up for your second-place boobs by flaunting them at every opportunity! Well you're not going to get away with it THIS time, Margaret Anne! Don't let the minx fool you, Mister! She's got NOTHING compared to these!"

And with an angry growl, the red head ripped open her own dress, sending buttons flying only to freeze in mid-air as they left the time bubble you had created. You have better things to look at than flying buttons, though. Patsy had made no idle boast. Her 21-year-old tits were indeed spectacular! They quivered as she breathed hard in anger and embarrassed pride, showing off her rack to her sister as much as to you.

"So they're bigger than mine!" the blonde sister exclaimed. "So what? So is your butt! Don't let her big bosoms distract you, Mister! You've got to consider the whole package!"

So saying, Peggy stood up and wiggled out of the remains of her dress, revealing her slender waist, slight hips and long, slim legs. Her carpet matched her drapes as well, you notice.

"You look half-done!" Patsy declared. "A real woman has curves and she knows how to use them!"

And as the redhead wiggled out of her own clothing, she began jiggling in a way that her less-curvaceous sister could never manage. But it might be fun to let her try.

Just based on their bodies, you'd probably go for Patsy, since a big pair of tits is hard to ignore. But of course, you don't really have to choose either. You'll most likely leave them both the age you've put them at. But there's no reason to let them know that yet. Not when there is such an interesting sibling rivalry to exploit! The only question is how?

"Ladies, I think I might have a way to determine which of you is most deserving of youth," you tell them. "Just a little competition."

"Competition?" the sisters ask in unison.