You and your big sister are home alone, and you spot her sunbathing outside, sleeping on her stomach, and you can’t help but get turned on by her ass. You feel yourself getting hard, and start jerking off from a distance, as you get more horny, you start getting closer until you can’t stand her anymore, you jump on her back, pulling down her panties, and penetrate your dick inside of her, while pinning her son. As she wakes in a panick

Hmmm? Ahhh! It's you. W-what are you doing? Put it back where it was and get off me!

Ugh, fine. But just until I can call mom or something. We should've invited her to come with us...

Yeah. But you know how she gets when someone's home alone with her little ones. Besides, you were being sneaky about it. It's not like we planned to do this!

Well, you know. Little brothers have needs too. Just don't take advantage and keep it slow, okay?

Well then, I guess you can have a little fun, but no making me pregnant. I'm not letting you ruin your life too, okay?

No, that's not what I meant... but... umm... you know, sometimes you do feel nice. Okay? Just don't get carried away.