The stranger calls

You get in the car and together put as much distance between you and your sister as possible. After forty minutes of silence, you park in the shade of tree in a distant park. You cannot even look at each other. Then your phone rings.

You know who it is before you answer, "That was rather brutal, don't you think?" there is however amusement in the stranger's voice as they talk.

Jessie grabs the phone, "we did what we had to you piece of shit! Now turn us back!"

"Well, it was certainly a good effort but will it be enough?" replies the stranger without changing their tone.

You do not think you could have been more stunned by the comment then if you had been slapped in the face. Words fail you for several moments as you open and close your mouth. Finally, you say, "what do you mean, 'will it be enough?"

The stranger laughs, "I made sure you two were fertile, I said nothing about making anyone else fertile."

The stranger's words slowly percolate in your mind and you only half hear what they say next. "the deal was you impregnate the next girl you see, however if that girl was infertile at the time, well that would just be too bad. You could pump all the sperm you want into her but at the end of the day you would still be a new mother."

The stranger hangs up before either of you can reply, not that either of you could in your state of shock. You had just **** your sister and it might have all been completely futile.