Some growing sisters, then.
Jessica has always been a sweet, playful girl for as long as you've known her. And almost as long as you've known her, she's been... quite tall for a girl. Some would think she'd be extremely embarrassed about it. Those people have never met your sister. Jessica loves her size, and it's definitely no secret that she wishes to grow even bigger. Your parents have used this desire to their advantage in getting her to eat her vegetables including broccoli, among other things, but they never did fully understand this desire: they always thought she wanted to grow up 'big and strong' when really she wants to be a huge giantess with bigger curves than most anyone has ever seen on a girl. Thankfully for you, she's always been fond of her sibling.
If she's younger, she's constantly pestered you to measure her whenever she has the slightest belief she's grown - which happens at least once a week. Imagine her joy when she finds out she's starting to grow. Now imagine that same joy on an increasingly bigger girl, who looks more and more like a goddess of sex as time passes. Who knows when or even if she'll stop...
If she's older, Jessica has prowled virtually every giantess-related site you can think of, and while has reluctantly come to accept her dream is just that, it's a dream that refuses to die. Luckily for her, sometimes dreams do come true! And of course the only things that could make her happier are if she keeps growing bigger and sexier, and if you're there to watch. Adult size: 6'1", C-cup breasts.
Olivia and Emma are sisters, with the former being the elder. For most of her life, Olivia has been tiny, but just about the kindest girl you can imagine. Emma is more normal, if a bit tougher due to her sister being a bit on the sensitive side, but still wouldn't hurt anyone without a good reason to. This changed when they were 12 and 10 respectively, as puberty decided to bless Olivia, letting her grow to 5'7" with huge (for the time) C-cups and a generally much improved figure. It's been a few years later, and both have grown, with Emma now at a respectable 5'3" with B-cups and Olivia at her ideal size of 6'2" with wonderful D-cups and a generally nicely toned figure.
Whichever sister is chosen, the story will follow from the perspective of the other sister. If Olivia is chosen, you as Emma will witness Olivia's growth spurts return, making her reach heights that no human has ever before.
If Emma is chosen, you as Olivia will behold your younger sister overtake you once more as she becomes a giantess. And because you're still so kind, you'll take it upon yourself to try and keep her calm.