Waking up with little sister in law in bed with us.

I give in to my temptress SIL with my wife asleep next to me

I got married at 18. The girl I married was 16. We started dating in HS and once I graduated I got a good paying job and asked her if she wanted to marry me. She said yes, so we got married that spring. She stayed in HS until she graduated.
She had come from a large family (9 kids) and had helped her mom raise the kids after her Daddy got killed working on the railroad. There were still little ones at home so we moved in with them as they had a big old house.
I worked her mom worked and her next oldest sibling (brother) had apart time job. Mom had a settlement from husband getting killed, got social security for the kids, and a widow's pension. So we could live quite well.
We all got along very well, shared our resources, and I liked it there. I had grown up super poor and my Mom was a single mother. So it was quite a change for me.
The problem was her younger sister who was 13 when we started dating. She took a shine to me from day one. At first it was cute, but her flirting got to be quite aggressive. My wife and mother loved to tease me about it as I was a little shy and would turn red quite easily. It got so that I told my wife she needed to get Cassie to tone it down. Ellie (my wife) rolled her eyes at me but she did talk to her and Cassie was better about it just not so open and aggressive though.
So we continued to live there and we all came to be pretty close. The younger kids looked up to us, Mom loved having us around. The only problem was her little sis Cassie. By now she was developing in to quite a beauty. She was 5'8" strawberry blond hair nice B cup breasts and a heart shaped ass. She also had very pale skin and green eyes.
When we had a B-day for her she made a silent wish and blew out her candles. People asked her what her wish was, but she just smiled and said "Can't tell or it wont come true" Then gave me a impish grin.
All during the party we had she was flirty with me, but not so anyone could really notice. Me and Ellie were drinking and getting a little drunk with her mom and the older kids and relatives. (yeah we were underage).
I went into the kitchen to get another beer. Cassie was already in there. She said "need another beer, Jon?" and reached in and got one for me, opened it and took a sip then licked her lips seductively. She handed me the beer saying "You don't mind I had a bit do you?" I said "Naa its your birthday" She came close up to me, put her arms around my neck pulled me to her and said "How about a Birthday kiss for the birthday girl?" then without waiting for me to reply pulled me in and kissed me full on the lips. Like I said I was a bit drunk, so I thought "I'm going to give something to remember" and really got my tongue in, running my hands down her sides, cupping her perfect little ass giving it a gentle squeeze. I felt her shudder and went "uhhh "into my mouth. We stood there kissing and of course I got a raging hard-on. I tried to back my hips away so she wouldn't feel it, but she backed me into the counter and pressed her self hard on to me, grinding a little. We came up for air and she went "WOW that's a real kiss" then we chuckled and I said "I better get back out there.
Later, after the younger kids went to bed, her mom, me, Cassie and Ellie started watching a movie on HBO. Ellie and Mom were soon fast asleep. I felt a small hand nervously rest on my thigh. I turned and looked at Cassie. We started kissing and I cupped her perfect firm breasts and she put her hands inside my shirt and we kissed for a while. Then I came to my senses and pushed away from her checking on my wife and MIL. They were still both asleep. I said "I think we need to go to bed now" Cassie gave me a pouty look" and said "OK i guess. thanks for the Bday present" I asked her "Was that what you wished for, earlier?" She grinned and with a little twinkle her green eyes said "Nope" then pecked me on the cheek and took off to her room. I put a throw over mom, woke and helped my wife get up and said "Time for bed sweety" she yawned and stretched put her arms around my neck looked up at me kissed me and said "You going to tuck me in?" Then we headed up the stairs to our room. When we got in in and she pushed me onto the bed, pulled my pants off then hers and climbed on top of me, straddled me. I slid into her so smoothly into her dripping pussy. She started humping me pleasuring her self, making sure my cock was hitting the right places. She came with a low little wail and her nipples got really hard which when I felt her pussy throbbing and gripping m cock I exploded into her. She collapsed onto me breathing hard. I started to fall out. She looked at me and said "Uh uh I'm not done yet" The got into 69 and began sucking on my messy cock making mm mmm mmm sounds. I started eating her pussy licking and sucking on her swollen lips and clit. Our mixed juices dripped onto my face. I took my tongue rolled it around her swollen clit and began going up and down it. When she started cumming It made me cut loose with another big load. She was cumming so hard her mouth came open I heard her gasping with every twitch of her cunt. She the curled up to me and went to sleep.
I woke up later, i wasn't sure how long how long I had been asleep. I felt a warm soft wetness on my cock. I thought She's really horny tonight, She swirled and licked around the head and soon I was so hard it hurt. She lifted off of me crawled up straddled me then plunged down onto me. I heard her go "aaahhhd yssss" I reached up to hold her tits. They felt firmer that usual, but I found her nipples and began squeezing them. She stopped little bit, waiting for the feeling of wanting to cum to stop. We kept doing that until finally we couldn't hold on any more and we both came together. She laid down on me rested her head on my shoulder. I pulled her up to kiss her. Then I saw the green eyes of her little sister. I startled and started to say something but she put her fingers on my lips and went "sssshhhh". I started getting hard again while I was still inside her. She whispered into my ear " I don't think you want me to go yet" and chuckled. I thought oh god what have I done now. But we started fucking again. She made me sooo hard I couldn't help it. My body just took over for me. When I came in her again she came in response to my semen filing her again. We laid there with me inside her until I began to shrink and fall out. My cumm ran out and down my balls. Then she very slowly got up. went to leave then turned and said that was my Birthday wish.