I was alone with my sister who has huge breasts in an open-air bath! And I never thought I'd get an erection just by looking at her naked body! I went to a hot spring with my sister on a family trip for the first time in a while, and I was surprised to see that her breasts were bigger than I had imagined! I didn't mean to look, but before I knew it, my eyes were always on my sister's breasts. They were so big and looked soft...