You walk into the living room to see your sister sitting on the couch stretching. As soon as she notices you she smiles and tell you how she is just stretching out her muscles after a nice workout and run. She continues to stretch as the two of you talk and you can't help but look at your sister's amazing body in her tight workout clothes while she does so. After a few more minutes of stretching your sister asks you if you could give her a little back and shoulder rub as a reward for all the hard work she has been doing with her dieting and exercising. You agree, so the two of you sit down on the couch and you begin working on her back. Even without her telling you, you can sense just how pleasurable she finds your massage. "I'm not trying to make things weird, but this feels almost orgasmic" she remarks, making your steadily growing erection even harder. Finally, she turns around, making you stop. She says that there is one more thing that she needs to complete her workout regimen for the day. She needs some more protein. Almost hesitantly, your sister tells you how she has read that cum is an amazing source of protein and nutrients. And she wants to know if you can give her your cum. She is okay with you just cumming in a cup and ingesting it that way. However, she has also read that it's best if you ingest it right away.... as in, swallowing it as soon as you cum. You can't believe your sister is offering you a blowjob! Nothing like this has ever happened before! But you're already rock hard and she knows it, she can easily see it, after all. And she has always found you attractive. You both want it, and it is for your sister's health. So, you pull down your pants and your sister begins to suck on your cock. She works her mouth up and down it. No blowjob has ever felt as good as this blowjob from your sister. It's even hotter seeing her play with herself, knowing that your sister is getting desperately turned on from sucking her brother's cock. She keeps on licking and sucking your throbbing dick until you finally explode into her mouth and she greedily swallows it all down. But even after cumming that hard, your cock is still hard. There's still more cum in your balls. Your sister could keep on sucking the rest out, but she is so turned on that she needs you inside of her. Her belly is already filled with your cum anyway. She climbs on top of you and rides your sensitive cock, sending waves of pleasure crashing throughout both of your bodies. After awhile, she instructs you to get on top so that you can get your own little workout in. You eagerly do so and resume pounding into her tight pussy. You're going to fill up your sister's pussy with cum just like you filled up her stomach, all while she cums hard on your big cock.