Background:

My wife, Emily, (26) and I (28) have been married for 2 years, and my sister-in-law (29) and her husband (29) have been together since high school. My SIL (let's call her Rachel) and her husband (let's call him Mike) had been trying to have a kid for 2 years but didn't have any luck. To help paint the picture, Rachel is a 5'1 brunette with a super tight body and small B cup boobs. Mike is also a relatively small dude, about 5'6. By comparison, my wife is a skinny 5'5 brunette with B cup boobs, and I'm 6'2, 215.

They went to a fertility clinic for help but unfortunately Mike was having fertility issues and they're unable to conceive. Having a kid is very important to them and this issue has weighed on them heavily.

They looked into possibly using IVF with sperm from a sperm bank but the bill was expected to be north of $12k, so they were desperate for a cheaper alternative.

Cut to:

About 3 months ago, Rachel reached out to my wife about a potential solution. Because we are all so close and it's so important to Rachel and Mike, Rachel asked my wife if she would be comfortable with approaching me to impregnate her. Rachel felt comfortable knowing that the biological father did not have genetic issues and they were generally happy with the idea of their kid having my basic traits (I'm not an idiot). Understandably,my wife was immediately against the idea since she was not open to me having a biological child with her sister or with the idea of me having sex with her. In fact, my wife didn't even tell me about this proposition for another month when Rachel made another attempt at convincing my wife.

1 month later, Rachel and Mike came to visit us and were staying at our place for the weekend. We went out to dinner and drinks and were having a great time when Rachel and Mike brought up the topic. It was my first time hearing of it and I was immediately excited and terrified at the idea. Ultimately I was good with it as long as my wife could be convinced. Rachel explained she was currently ovulating and wanted to see if my wife would reconsider potentially trying something that weekend while they were in town with us. The key was that Mike was very open with how he was ok with it even though he would be the one raising a child that was not biologically his and also dealing with the idea that his brother-in-law had sex with his wife.

Reluctantly, my wife agreed to go with it knowing how important this was to her sister.

After dinner, we went back to our place and continued drinking a bit to ease the tension. Rachel and Mike were pretty drunk at this point and starting making out with each other on the couch. My wife and I looked at each other a bit confused but went along with it and went to our bedroom alone.

We figured they were good to do their own thing for the night so we close the door and started getting into it. I took off my wife's dress and heels and she took off my shirt and pants. We were laying on the bed kissing with my wife just wearing her black bra and black lace thong and I only had on my boxers when we heard a light knock on the door. Before we could even respond, Rachel and Mike opened our door and walked in. "Knock knock, can we come in?" Rachel stumbled in as she was clearly a bit tipsy and Mike had his shirt unbuttoned.

My wife and I were already in the mood so we moved over a bit on the bed and continued our foreplay. Rachel and Mike took off their shoes and hopped on the bed. Rachel jumped on Mike and straddled him while they made out. I couldn't help but keep peeking over at what they were doing. Mike then removed the straps from Rachel's red dress from her shoulders, revealing her beige bra and Mike began kissing Rachel's neck moving south. Rachel stopped and pulled her dress off over her head revealing her cute butt and nude colored thong. It was super apparent at this point that they wanted to try getting Rachel pregnant that night. I was still confused though if Mike wanted to be present for that, and I definitely didn't think my wife would want that mental image.

"Em, are you good with our plan tonight?" Rachel asked.

"Sure, if that's what you want. Do you want me and Mike to go?"

"No no, Mike thinks it'll be less awkward and less sexually intimate if you two stick around to make sure it's purely business."

This got me super turned on and I was convinced I wouldn't last more than 30 seconds, which would be a huge win for everyone. Both couples went back to making out and undressing each other, except my wife left her underwear on. Mike took off Rachel's bra, showing her beautifully tan-lined tits. Her skin was generally tan but her tits were still pale. Her nipples were almost identical to my wife's, small and pink, but very pointy.

What happened next got me slightly concerned...Rachel took off Mike's boxers to start blowing him. That wasn't concerning by itself but Mike had a surprisingly small penis, probably no more than 4 inches. Rachel and Mike had only been with each other ever and Rachel was already a petite girl. I'm not by any means the biggest guy, but my dick is a solid proportional 7 inches relative to my size. This was not discussed at all, and my wife and I looked at each other with a mutual understanding that this would be a shock for Rachel. "Hey Rach, before you commit to anything you might want to take a look at what you'll be dealing with next."

Rachel quickly stopped blowing Mike. "What do you mean?" she asked. I took off my boxers to show my hard 7 inch dick. "Fuck..." she blurted out. "You'll be ok baby," Mike reassured her and started pulling her thong down and off her ankles. Mike started going down on her and fingering her to make sure she was properly warmed up.

At this point, 3 of us are totally nude and my wife still has on her underwear. She takes off her bra and starts blowing me to help make sure I cum quickly once the penetration starts. I'm so turned on this point, I need her to slow down or I'll ruin this whole thing. "Rach...he's ready when you are!"

"I'm ready I guess," and Rachel let out a big breath to relax herself. "Can I start on top so I can control the speed?"

I laid down on the bed and Rachel started to straddle me. She grabbed my dick with one hand and used the other on my chest to balance herself while she positioned my dick to be inserted. My tip and next few inches slid right into her soaking pussy and she put her hands on my chest to help her control the depth. She went up and down slowly and gradually going deeper. She was letting out gradually louder and louder moans that were clearly a combination of both pleasure and a bit of pain. "Wow I think it might be too big, I don't think I can do this!" Mike again reassured her she was doing great and she'd be fine. "Maybe try another position? Let him get more leverage and do this thing," Mike proposed.

Mike laid down on the bed stroking his dick and Rachel repositioned herself into doggystyle. Rachel started blowing Mike and I got behind Rachel while my wife continued to watch in concern. Rachel stopped for a second to reach between her legs to position my dick back in her pussy. I grabbed her hips and slowly began going deeper and deeper until I was finally all the way in. Rachel lost control of her arms and now face down, ass up with her head resting by Mike's legs. She let out a loud scream into the sheets and her legs started quivering. I came instantly, pumped 4 loads in her and pulled out.

"Thank you," Rachel muttered with an exhausted breath.