My wife and I are having a happy life, she is eight months pregnant with our first child. Just when I thought this happiness would last forever, my sister-in-law Tsubasa appeared. She is extremely free-spirited, having traveled for six years, so I have almost no memories of her. But she is too attractive! When at home, she doesn't wear a bra, and even the pants she wears can reveal the mysterious "gap" of hers! Because my wife is pregnant, I have not been able to have sex for a long time, accumulating sexual desire causing my penis to quickly harden against her hot body. She also noticed and approached me, touching me and starting to seduce me even when my wife was nearby! And so, in the following period, we kept having sex. She always lets me release all my sperm inside her, then lets me rest for three days to accumulate more sperm and release it all inside her again...