C.R.
there are always going to be kids around our kids that do things we don't approve of. Speak with your niece and explain your concerns and tell her that your rules are to be strictly enforced with your son. My kids will ask(and so will yours as he ages)"Mommy why is that kid...(breaking one of our rules)..." and my response is different families have different rules, his mommy is in charge of him and I am in charge of you...I have explained to my kids what I feel my job as their mother is...to keep them safe and healthy, hopefully happy and to help them become strong boys/men who will hopefully make good choices as they get older...this isn't a sit down conversation that we have it's bits and pieces of each day...the answer to "why I have to go to bed now" is "so you can grow and be well rested for tomorrow"...and your son will come home with things/words that you don't like...just take a deep breath and don't over react to it....keep it simple while he's young with a "we don't do that" but as he gets older tell him why we don't do that....good luck!