Yes, ma'am!

Immediately after you affirm her request, your lovely sister giggles in triumph. Taking her time after she lets you back into her mouth, she ups the anty on the pleasure scale. Slidding a pair of fingers inside of your rectum, you gasp as she finds your prostate and teases the hell out of it. What in the hell was in those books that she was reading? No matter, this is the best outcome you really could hope at this point.

Feeling your legs wobbling like jelly, you manage to squeak out, "I'm going to cum!" Before unleashing a load of cum into her mouth.

Your sister nods, apparently grateful for the warning and drinks all of your warm seed before gently pulling away with an audible pop. Giving your still erect cock a kiss on the head, she resumes stroking it while sitting there in thought. She has been feeling herself while sucking you off and there are clear signs of arousal between her legs.

"Tell you what? Since you've been such a fast learner, I'll give you this one for free," she turns around and gets on all fours, "hurry up and fuck me." She orders.