Hey! I’m not sure if he was her uncle or a family friend but I remember the basic plot.
Older man goes the the bathhouse after a long day his (I believe) niece comes in to take a bath too. He ends up washing her and then they have sex.
He remarks about how he would was her when she was a child and it wasn’t weird but it’s weird now.
I looked up everything from Uncle to Niece to Bathhouse to Onsen and nothing came up.
Thanks