On a family trip, my sister takes me to an open-air bath. I don't usually pay attention to her big tits, but I get a boner when I see them! I try to hide it, but my sister is so defenseless that she curiously approaches me with her sexy body. I can't take it anymore, she'll be put off if she finds out! But when my sister notices the state of my genitals, she reaches for my erect penis...