Do you think my sister is over reacting at me for letting her 9 yr old son watch scream and aliens?

Well she did ask you to baby sit and if she had any other instructions she should have gave them, she was very detailed about drinking, smoking and skanks so she should have been just a little more detailed about rated R movies. Kids are very impressionable and they pick up on everything, my son is not allowed to watch rated R movies or play with toy guns BUT I can not control what happens when I am not around (which is a bummer but it's reality). Your sister needs to lighten up and realize that everything and everyone do not operate and or think like she does and (I'm guessing you have no kids) if you have no kids why would she expect you to be the BEST baby sitter in world. I would have been a bit irritated but happy that he was feed, clean and in good health. She'll get over it eventually