You pull the wand out of your pocket and think "I wish my step-sister's breasts grew five cup sizes. You put the wand back into your pocket and crouch in front of her, watching her baggy t-shirt. At first, you see nothing, but soon enough her shirt starts to fill out. Her breasts start to push against her shirt at around a G cup, and gently keep pushing. Her shirt begins to strain some once she reaches H, but is pushed to the limits at I. She moans a little, with a hint of pleasure in it. You watch as her nipples become erect, pushing out her shirt like thimbles, because they had grown with her breasts.
You reach for her right breasts and squeeze her large nipple through her shirt. She looks down, making you shrink back in fear of getting caught. Instead, she smiles and takes off her shirt. You stare in amazement as her breasts flop down, jiggling enough to cause them to bounce a little. She holds them up, weighing them in her hands, and starts playing with herself. "Oohh," you hear her say. "I don't know how my breasts grew, but I love it!"
She massages her breasts, moaning in pleasure as she grabs and gropes. You reach out and squeeze her left nipple, making her gasp in pleasure. She slowly moves her left hand down, sliding past her stomache and reaching into her short short. You watch as she pushes her fingers under her panties and begin to masturbate.
You continue to watch her masturbate, sliding her hand in and out of her clit. You practically drool at the sight of her groping her large boobs. You continue to watch for a little bit before deciding to do a little more.