Step sons girlfriend

I have a 16 year old step son. He has been seeing this girl for a few months and she comes around on the weekend to hang out. she is a very hot girl. great little body. anyhow, they asked if they could use the hot tub and my wife and i said go for it. I really wanted to see what she would wear in the tub. I had a feeling she would be in her panties because she had nothing with her when she came in the house. I never did see what she wore because i didnt want to be obvious with my wife around.
This morning i went to my basement were the kids hang out and found a bikini top and a tiny green thong that must belong to the girl. my c*** instantly got stiff but i didnt j*** off.
My wife will be out of town this weekend and i pray that the girl will come over so i can see her in that little thong. if i do ill j*** off for sure