I pushed my way into her, waking up mom at the same time. Anna, my sister was screaming in pleasure as I thrusted into her. Gone was any shred of inhibition. She rocked, breasts heaving as she was lifted to extreme pleasure.
My mother popped up and saw what was happening. "NO, Anna baby you have to hold don't do this please, not to my baby!"
Anna bounced up and down, uncaring of the scales spreading down her legs, a beautiful shade of purple. "Stop" mom pleaded "you'll be a mermaid if you cum, don't become like me!" She begged.
"I WANT TO, I want a tail, oh give it to little brother, make me a mermaid, get rid of my legs!"
I smiled as I pressed her legs together, watching as we both reached completion. Her legs, now all but covered in scales merely awaiting one final push.
"Cum in my brother, cum in my mermaid pussy, PLEASE MAKE ME YOURS!" My sister screamed!
I moaned and shot my load deep within her, feeling in go deep inside her. She arched her back, breast heaving as she screamed "YESSS"
At the top of her lungs. Her feet fanned out into a beautiful tail, as gills appeared and her ears pointed.
"I am yours master" she moaned at me as Mom looked on horrified.