Need advice on parenting in front of other parents

I'm currently 15 weeks pregnant with my first. My concern is my sister never seems to discipline/correct her 4 year old son for anything. He will hit, yell, and talk very disrespectfully towards people yet is never corrected. He's never made to apologize or say please or thank you. He will eat a few bites of his dinner and say he's done then ask for dessert and munch on candy the rest of the night. He brings toys, game boys, iPads to the dinner table which distracts him and causes my sister to have to feed him herself. I don't plan to tolerate this with my child, but how am I supposed to discipline them for something their cousin is getting away with?