Your little sister is a lot younger than you... or rather, younger than the old you... but your mom couldn't have had a kid when she was two, so...
The program displays your little sister, and you learn she has reverted back to fourteen years old. And she is very cute, just like you remember when she was. You remember how, while you were in college, your buddies kept nearly getting themselves in trouble because they didn't realize she was only a ninth-grader. This little jailbait is ready to do it all again now...
Sis never married, and she wasn't happy with her life, so you decide to making her fourteen again is just fine.
She inherited the same rack from your Mom that you did, this rack that you now notice you've been absent-mindedly groping. You're feeling a little horny, and your bra is feeling tight. With another giggle, you pull off your shirt and after a little fumbling, your bra. Freed up, your breasts feel great in the air-conditioning, and your nipples are hard, and actually slightly sore. You start rubbing them, then return your attention to your sister. Her bra size, 34C, is cool and all but just not brag-worthy enough for Dad. You like the idea of giving him a daughter who could easily end up a centerfold. You tweak Sis up to 36D, but it doesn't have the right effect. Making the number bigger is making her plumper instead of growing her breasts. Although it's hard to tell, under the clothing.
You wonder if you can check how she looks nude. There's a menu to select clothing, and you try that, but a window comes up warning that this will remove her clothing in RL! Not what you meant, although with another giggle, you sex up her lingerie before moving on.
Finally you find you can hide the clothing instead of remove it, which doesn't cause a warning. Now she's standing in front of you naked, and yes, a little heavier than you want, as you thought. You put her back to 34, then try making her DD. It looks crazy hawt on her, so you try DDD, then E, then F!! It looks amazing, but also kinda gross. You have a feeling those things flop around pretty bad. You crank it back to 34DDD, which you check out from every angle. It seems a little floppy still.
Despite that, she's now some serious masturbation material, and you're getting hotter. Almost without a thought, you strip off your jeans and panties, and find with your finger that you are already slick with pussy juice. You spend a couple minutes learning how to finger yourself, then screw yourself for a while to the busty girl on the screen.
There's a help screen, and working one-handed you try it, cause you don't want to decrease these massive boos, but you don't know how to fix the sagginess. It takes searches on 'breasts' and 'sag' and a few other key words, but you finally find what you are looking for under 'mammaries'. You find its possible to add some firmness by making her more into fitness, that's not enough. Then you notice the help mentions, 'when menu choices do not produce sufficient results, try the search suggestions tool'.
You find it and type in, 'make large tits firmer'.
A list of possible choices comes up... there's actually a lot of them, in a scrollable window. Some of them were about finding optimum combinations between size, fitness, etc, while others suggested changing the breasts to implants.