my stepBROTHERS big dick turned me into a taboo slut

Your stepSISTER is just trying to relax and enjoy getting into a nice hot bath after a stressful day when you walk into the bathroom to wind her up. You know the door lock broke last week and you also know that it'll really annoy your stepSISTER if you hang around the door for long enough to make her think your going to prank her or something. She tells you off, begs you not to do anything and closes the door in your face. Inside the bathroom, she is standing on the other side of the door, slowly and nervously getting undressed. She smiles as she thinks about you seeing her naked- (maybe it would be quite funny) and climbs into the bath. She calls out to see if your still there but you don't answer, which weirdly enough makes her pissed off that you'd even dream on missing out on a rare opportunity like this. After she is sure you are gone (you aren't really) she slips her hand in between her legs and starts to masturbate. Lost in the moment, she doesn't even notice you sneaking in and, as you stand there watching her, you can't help but get turned on at the sight of her nude (even if she is your stepSISTER) .. suddenly, she opens her eyes and catches you. After quickly covering herself up, you both laugh it off, but it isn't long before conversation turns to how she's seen you naked before and when you ask her how big your cock was and she says "6 inches" you know damn well she's lying. She doesn't believe you when you say you've got something substantially bigger than a 6 inch dick so she asks you to prove it. When she sets eyes on your secret monster, she is fast to ask if she can touch it.. you are reluctant but give in as she is so fucking adorable how could you say no. Apparently it's the biggest one she's ever seen in her life! She then asks if she can move her hand up and down.. just a bit.. just for a few seconds.. then she's practically begging you to let her suck it.. "we shouldn't be doing this" she says "this is so twisted, nobody can ever find out about this" .. you worry that you'll cum from how good her mouth is on you, but then she tells you she wants you to fuck her.. what the hell.. this was only supposed to be a joke..  you can't fuck your stepSISTER.. you mustn't.. even if she begs.. right?.. RIGHT?