Your sister-in-law

You have been in a loving and care-free relationship with your girlfriend Dana. She has a great personality and looks great. In fact, you've always been something of a boob man and the first thing that caught your eye when you met Dana was her large D-cup natural breasts.

As you got to know her well enough to get to meet her family, you learned that Dana had a sister, Caitlin, who was roughly four years older than Dana. Caitlin was even better endowed than her sister, and you feasted your eyes on her huge breasts which you estimate to be at least G cups. Caitlin slowly became your go-to fantasy when masturbating and wanting something else than the usual big titted porn stars. Hell, thinking about her felt so much better, because she was an actual living, breathing person who you got to see in real life on a regular basis.

What Caitlin had inside her sweater, she seemed to lack in personality if you compared her to Dana. She was something of a klutz, which perhaps explain why she had managed to stay single up until her 40th birthday, which was today. You and Dana arrived at Caitlin's small apartment where the birthday party was held. The only other guest were Caitlin's former study colleagues, all female and nerdy. Come to think of it, you'd think she has never had any male acquaintances. Maybe she's a closed lesbian.

The party was pretty mediocre, and the other guest soon left. You, Dana and Caitlin sat at the table, drinking. Dana said: "What's up, you don't seem like a happy birthday girl?"

"Well, this just keeps reminding me that the clock is ticking. I'm 40 already. I'll never find a man and get babies..."

"Have you considered getting a donation? You don't need a man if a baby is all you really want..."

"I've been thinking about it, but it seems somehow... odd. Plus it's expensive and after all you'll never know what kind of donor you are getting."

Dana thought for a while and then laughed. "Why don't you just get a donation from John? He's top notch dad material!" The three of all you laugh, but then Dana continues: "I'm actually serious! He's practically never been ill as long as I remember! You'd have super healthy kids. Plus, they'd have John's good looks", Dana said, poking you in the ribs.

The two sisters ended up egging each other on, thinking of reason after reason why it is the only possible way for Caitlin to get what she wants. They both were now actually begging you to donate your sperm to your girlfriend's sister. Of course you agreed.

The next week, you get a text from Caitlin saying she just made an ovulation test which shows that she is fertile now, so this would be a perfect day to perform the donation. You get a slight boner just from reading the text. Of course you'll only be jacking off into a cup in Caitlin's bathroom but the though of your sperm ending up inside her is turning you on like nobody's business.

Later that day, you arrive at Caitlin's door. She answers the door in her nightgown, apparently not wearing a bra underneath which gives you a first great view of her nipples. She notices the cup you are holding in your hand. "What's that?", she asks. "Oh, I brought my own cup. So you don't have to soil one of yours...", you stutter. Caitlin pulls you inside and into her bedroom. "You won't need a cup, silly", she laughs, "I want my donation as fresh as possible", Caitlin laughs and pulls off her nightgown, revealing breasts almost as big as her head.

"Come on, let's get you nice and hard! I know you like to look at my tits. Feel free to touch them as much as you like."

You stare at Caitlin, not believing this is actually happening to you.