That did not happen in the movie. Maybe the alternate ending but that so did not happen on the version I just watched on Netflix. Dude, his sister was there... that's just perverted. There is a time for sex and there is a time to hug your little sister and let her know everything will work out.
When did this happen? Maybe I zoned out, but the ran out of the asylum to Flagpole Sitta and then they were pulled over by the cop? Did I miss a whole scene?