I went on a hot spring trip with my nieces, whom I hadn't seen in a while. They caught me peeking at them while they were changing and invited me to take a bath with them. As we washed each other for the first time in a while, they noticed my erect penis and pretended to wash me, but instead grabbed it and stroked it. My niece is grown up and in heat now, so it's okay for us to have sex alone in the bathroom.