SW-1030 - Mixed bathing with grown nieces during a hot spring trip! A lewd girl secretly holding my erect penis. I'm going to secretly put it on when no one is around!

A hot spring trip with nieces I haven't seen in a long time. I was caught peeking while they were changing, and was invited with "Then, Uncle, let's take a bath together!" While washing up after a long time, I noticed my penis was erect and pretended to wash it while holding and stroking it. Now my nieces are grown and sexually mature adults, so it might be okay to have sex with them alone in the bathroom?