As it's rather after the fact this is something of a moot point, but your best course of action would have been at the time to stand up for those girls, take your cousin aside for a second and give him the verbal dressing down of a life time, implying among other things common courtesy and respect towards women, the basic mannerisms involved in being a guest of someone else, the excruciatingly painful consequences of not complying with the afore mentioned as well as your dad telling his dad what an utter asshat his kid is, and maybe that'll cost him his shiny car.
Either that or you deck him
sooo hard that he still feels it when he's 35.
