My (41M) single SIL (36F) (wife's sister) was housesitting for us and had helped herself to some wine while texting some potential suitors; my wife and I got home late and tipsy, and my wife passed out.

When I walked my SIL to her car we were a little flirty about something that happened earlier. Long story short, we wound up making out like teenagers in the back seat of her car. Clothes stayed on, and we touched each other over them. Eventually she got on top of my lap and we grinded while making out. After a few minutes, she had an orgasm, which set me off and made me cum in my pants... which made her cum again. We kept making out a little more, and then she had to go, saying it was a one-time thing.

The positive update beyond that is that she's talked to my wife on the phone 2x since this happened and hasn't said anything.