Aneshota!

Full title
Aneshota! -Your Spunk. I'll Suck It Until There's None Left-

* An ADV about Yuzuru's daily life as the plaything of his older sister *

Story
Spuuuurt spurt spuuurt--!
"I'm gonna..... die...."
Protagonist Yuzuru was a young guy with little sexual experience.
"You're.... killing me....."
He had an older sister that he cherished.
"Is this how I die? On your hips!?"
"Hahaha! You sound so horny, Yuzuru! Keep moaning!"
"N, no! Hhnnngh! Aahhhhhhhh--!!"
"Just like that! More! You're a good boy, Yuzuru. So cute."
"--aahhghhh?"
Only half an hour ago she was a different person.
She'd transformed into some kind of c*ck devouring demon.
"S, someone help meeeee!"
"Shhh, Yuzuru! Calm down!"
"Yes ma'am."
She always scolded him. He always did as told.
But his lower half was getting no reprieve.
"I want to play with you more. Will you help me out, little brother?"
"Ughhhh, hfwaaaa, fwhaaaa..."
"Please? Yuzuru?"
She cooed to him with that sweet voice.
In the end, that day was the first of a very strange life as her sex pet.
Will Yuzuru ever be able to take back control of his penis!?

Official Site:
http://softhouse-seal.com/product/093-aneshota/index.html