Investigate the master bath

I move to the master bedroom and notice the door is cracked open. At this point, I can hear the water running in the shower and a woman humming. I step into the room and move to the bathroom door which is cracked open as well.

Peeking into the bathroom, a woman is in the shower. Through the pebbled glass shower door, I can see that she's leaning against the wall with the showerhead in her hand, using it on her groin. She's not humming, she's softly moaning as she pleasures herself. Based on what you can tell of her hair and body, you think this is Samantha, your sister-in-law.

What do you do? Watch? Join her? Leave?