I wouldn't try and compete with your sister, just stay the neutral aunt. Eventually life will teach this boy respect, he may need you more later in life. :-)
I am not sure an adult should judge the actions of a 9 or 10 year old based on adult interactions. Kids this age are still maturing and many are just not yet ready to make responsible or proper etiquette decisions. Maybe he needs more attention paid to him? :idunno Many times kids act out...