My parents were planning to go on a hot spring trip, but I decided to go with my sister instead! I thought it would be a hassle to be alone with my caring sister who can't let go of her brother, but it turned out to be a god-like development! My sister crawled into my futon when we went to sleep, and she had a bad sleeping posture, so her yukata was open and her big breasts were completely exposed! What's more, when she turned over in her sleep, her big breasts were in front of my face! I got so excited by the super close contact that I couldn't breathe and got a full erection! When my caring sister noticed this, she tried to calm my excitement... and we ended up having a forbidden incestuous relationship!