You go after her.

You rush after her. She catches you following her and eyeing her ass and stops suddenly to turn on you. Your hard dick pokes her in the ass as you collide into her.
"Poking your sister's butt with your dick? How would you like it if I poked you with mine?" She punctuates her remark by wagging the end of her huge and now hard cock at you. You get hit by flying drops of precum.
"I might enjoy it as much as you enjoyed being poked by mine," you tell her as she looks in horror at her fluids on your face. You pull down your shorts, grab your cock and poke her with it again so that your arousal leaves a wet spot on her belly. She turns to go, but you grab her hips and pull her ass back to your cock. She only moans at the contact.
"What are you doing? Are you thinking about fucking me?" Cassie seems only half-appalled.
"You can use your cock on me, too," you whisper in her ear.
"What? Seriously?"
"Yup, it's only fair."
"I didn't even consider that anyone would want to be with a freak like me," Cassie tells you.
"Well, you are my sister and we are both freaks. I got a thing for putting big things in my butt and you have one." You try to untie her top in the back.