Quoting abcde:" see we dont have the greatest relationship. so i tend to just keep my mouth shut to keep the peace. but ugh its really bugging me.i feel like the stuff he is saying came from her"
well dont be blunt.. ask a few questions.. or can you talk to your nephews dad instead of his mom.. maybe say hey can you guys tell him to stop saying this stuff to DS. what you guys do works for you but when it effects DS its causeing problems.. being his mom you have the right to choose what he watches and what car seat he has.. not them.
that or just explain that your nephews opinion is his and your are yours.. that he just needs to ignore him and listen to you.. tell him hes not a a baby and doing certain things wont make him one either.