Finally did it with my SIL

When my wife and I were dating, I used to admire her younger sister's great ass and body. She was a real cutie. However, Becky got married and had three kids.
Now, 20+ years later, her marriage is not doing well, and she comes over to visit my wife just to get away from the stress.

For the past several months, however, our casual "Hi, how are you" hugs have become a quick pat or squeeze on the butt, and most recently, Becky's return hug included running her hand up and down my chest, with a pat on my butt. It was like we both realized what we wanted to do, but had never talked about it.

Last week, she stopped by while I was working at home. The kids were at school, and my wife was at work. I let Becky in, simply expecting her to say hi and go do some shopping while she waited for my wife.

She stepped inside, and we gave each other a hug. My heart began racing at the thought of being alone with her. Our hug lasted longer than usual, and I gently moved my hand down and patted her bottom.
"nice butt, Beck", I said. "Been working out?"
"Only at work", she replied, "I stay on my feet all day."

Getting a little bolder, I gently squeezed her butt, and she pulled closer. All of a sudden, without a word, we drew together and kissed.
And kissed deeply. Becky threw her arms around my neck, and I pulled her close. We continued kissing passionately, and finally broke the clinch.
"I've always wanted to do that", she said. "I know it's wrong, but I haven't had any attention from a man in months."

"Becky, I have to admit that for over 20 years I've wanted you. I love your sister, but still, you have always turned me on." I continued, "Beck, if you want to make love, I would love to please you and make you feel good. It's up to you."

Becky took my hand and led me to the couch. She turned to me, and started taking off her sweater. I helped her undress, slowly working her blouse off, unbuttoning her pants and taking them down. She stepped out of her pants, and removed her bra. Becky's tits are bigger than my wife's, even though my wife is 5 inches taller. She had a scar on her abdomen from the caesarian section, but her belly was firm. I knelt down and slowly pulled her panties down, revealing a soft, red-haired bush.

I stood and began undressing. Becky helped me with my jeans, and she followed my pants down to the floor. Pulling at my underwear, she smiled at my growing erection. Unlike all the stories we read of how guys have this 9 1/2 inch cock and 5 inches around, mine is right around 6 inches, and decently thick. What's most important, I get good strong erections and am not a premature finisher.
Becky stood up and drew close to me. I put my hands on her bare ass, and she leaned up to kiss me, with one hand caressing my stiffening prick.

We moved down onto the couch, and I began kissing and sucking her breasts, Becky's breathing was getting faster, and I swirled my tongue around her pencil-eraser nipples. Becky pulled my head to her left breast, and breathed for me to suck it.

In the meantime, my left hand was exploring her pussy. Her lips were not real large, and her clit was easy to find. Becky's pussy was getting wetter each moment. I could hardly wait to taste her.
After a few minutes on her tits and more kissing, I moved down her stomach, kissing my way down. She knew what was coming and silently parted her legs, waiting for my arrival at her cunt.
When I got there, I kissed around her pussy, the inside of her thighs, and gently moved in to tongue her labia. Becky's hips ground at me to put more pressure on her pussy. I pressed in harder with my mouth and tongue, and began licking her clit up and down.

Becky's eyes were closed as she moaned and thrust her hips. She squeezed her tits and pinched her nipples as I ate her delightful pussy.
After about three minutes of constant attention to her clit, Becky's body began rocking. Her breathing got faster and faster, and she grabbed my hair, pulling my tighter into her cunt. Her hips began heaving and bucking, and, finally, Becky stiffened in a wonderful orgasm. Her orgasms were intense like my wife's, but my wife never says anything when she cums. Becky announced her orgasm with barely more than a whisper, "I'm cumming. Mike, I'm cumming! Ohhhhh, I'm cummingggggg!"

After her orgasm subsided, she looked down at me and smiled. She motioned for me to move up, ready for me to enter her. With her ass on the couch, and her feet on the floor, I kneeled between her legs and she watched as I slid my cock into her. I did it slowly, as this was the first new pussy I'd had in over 20 years. Becky gasped a bit as I entered her, and she opened her legs a bit more for me.
I began stroking into her. Her pussy was like creamy velvet inside. She was hot inside, and it was absolutely amazing. I'd forgotten how good it was to enter a woman for the first time.
I was so excited I had to pace myself so I wouldn't cum too fast.

I fucked Becky for several minutes, and then we moved up more onto the couch. I ground my pubic area into hers while I fucked her, and she said she was about to cum again. I sped up my thrusts, keeping in time with hers. Becky threw her legs up and clinched me tight as she had another great orgasm. 

I could not hold out any longer, and told her I was about to cum.
"Cum in me." she breathed. "I want you to cum. Oh, this is so good."

My orgasm built itself from inside all the way down my prick. I came hard, my prick throbbing in absolute ecstasy.

We lay there a moment, with happiness in our eyes. No guilt, no shame. We got up to go clean up, and took a quick shower together, caressing each other and enjoying a tender embrace.
We talked about what had happened, and we both would like to do this again. She doesn't want to hurt my wife; neither do I. Becky and I are not in love; we both just needed great sex, and that understanding will make this thing easier.