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Some guy is at his girlfriend's house and is sleeping in a room by himself.

Suddenly he notices the friend's sister crawled under his blanket and starts sucking his dick or whatever. Then she starts putting it in her pussy.

Suddenly his girlfriend comes in to give him a good night's kiss. While he's kissing her, he creampies his girlfriend's sister hiding under the blanket.

Before the girlfriend leaves the room he asks here "Does your sister ever act weird?" or something and she just says "No, she's just a normal girl." or whatever.

I read this years ago but I'm into this kinda "cheaters hiding their misdeeds" stories and would like to find it again.

Anyone knows what the name/author of this doujinshi is?

Pic unrelated.