If you get aroused to non-consentual incest porn, you probably shouldn't tell anyone because you have a serious personality disorder and you probably come off weird already and have no idea. Full Scene + Moar: HERE
I like to believe this has less to do with his cock and more to do with her vagina. Some girls are just extremely sensitive down there, and I think that even a 3 inch pecker such as my own could produce the same results. Don't pity me for having hope.