Yes yes yes! I agree you should tell your sister In law to teach her kid ASAP.
i regret being shy and scared to scold my cousin last time now, i really regret.
He is SO spoilt!
He is now 11 and his mum still treating him like a baby.
Tell him when to drink water when to go toilet..
And even bring change of clothes for him + many others.
He is very spoilt dont know how to behave apporiately.