She tries to bargain with you.

You are very uncomfortable with the whole situation. Your sister being on her knees happily obsessing over your now giant cock and the fact you can feel yourself get aroused from her touch wasn't helping. You need to put a stop to this whole thing now before it gets even more out of hand. Yet before you can do anything to get her off your dick your sister says "Okay hear me out, what if I show you my boobs if you let me just play with your dick a bit, you can even feel them up if you like I saw you staring at them this morning? Cmon bro what do you say?" She then takes her hands from your dick and grabs her boobs which are each bigger than her damn head, and gives them a little lift. The pathetic excuse of a tshirt barely holding those impossible chest beefers in as her soft perfect flesh practically overflows from it. This horny beautiful beast that is now your sister stares right into your eyes with a joyful smile silently begging for a yes.