Maybe all the stuff your friend's sister took to make herself grow and to make her milk is still swirling around inside her body. Maybe some of it even made its way into her milk and is now rushing down your throat. The arousal surging through you and your racing heartbeat make them take hold even faster than it did for her and, as you suck on her fat tits, a pair of your own start to bud and grow.
You can feel the fabric if your shirt brushing over your swollen, sensitive nipples and you almost cum as you thrust your hips into her hand. She just thinks that you're really enjoying yourself so she pushes her breast even more against your face and even more of her transformative milk rushes down your throat. It doesn't take long for you to reach b cups, c cups, Ds and DDs and on and on. You grope her with one hand and yourself with the other. It's getting to be too much!
What took her months of supplements and care is only taking minutes for me. Maybe her milk had a concentrated effect. Maybe my arousal and enthusiasm made it more impactful on me. Whatever the case, my own breasts were starting to swell.
I would first start to feel a sensitive tickle as my nipples make themselves more prominent against my shirt. That sensation would soon heighten into full on pleasure as it became clear I had a small set of breasts. Moaning a bit as I continued to suckle her breasts. My own breasts starting to pull up my shirt as my breasts grew to average size. Her insistence for me to drink more only encouraged me to drink faster. Not that I didn't want more of her milk, I was already planning on drinking every last drop.
Within another couple minutes my breasts were large. My shirt was tight across my chest. The hand that was groping her ass now became preoccupied with my own growth. As my own hand explored my sensitive breasts, I'd moan louder against her nipple. In between each moan I'd pull in and drink as much of her milk as I could. This only fueled more growth and pleasure from me.
I'm sure I'll be leaking within a couple more minutes of this. However I think my shirt is going to tear away long before then as my breasts reach huge territory. Still not nearly as big as hers but I was catching up. The thought of pressing my own breasts against hers combined with her divine touch on my cock, my mouth full with her nipple and sweet cream, and my own sensitive breasts, I couldn't hold back anymore. I'd start bucking my hips into her hand as I'd start to shoot ropes of cum.