We were about to start trying for a baby... Our lives had stabilized, and we were diligently working on making a baby every night with my wife. One day, my wife's younger sister, Fumino, came to stay at our house for five days. We kept up our intimate moments, thinking it wouldn't be a bother... but it seems our noises were leaking out, and Fumino looked like she couldn't sleep. From then on, we started to notice each other, and we began having relations in secret, unbeknownst to my wife. As time went on, our encounters became more frequent, and all the sperm we'd been saving for making a baby with my wife ended up being used for Fumino instead.