This issue has been pissing me off for a while... How would you pull a nephew aside and tell them to stop taking the piss!??
Long story,,,
My nephew and his mum are absolute piss takers. His mum works a low paying job but brags her distant family has money, and if he doesn't succeed in education, he can go work for them in Australia. She's given him a debit card and see's all his spend and often goes ballistic. He eats places like Haidilao with his friends and settles the bill.

Whenever they go out with us, it's 3 courses and rather than eat the plate, they'll leave a load of food behind. Drinks they'll order wine and cocktails, whereas everyone else is on water (driving), soft drinks or a beer.
Recently on nephews 18th. My sis asked him what he'd like for a present. He straight up said he wanted VR googles or Mac Book Pro. WTF!!

I rang her and said just give him £50 or get him a clothes item. He got his mum to buy him a Mac book, which no doubt will be tossed aside in a month or so. He also wanted to celebrate with his friends (no family) at a City roof top bar. His mum put an end to that fantasy

The other day my niece took us all to a restaurant for her bday. (She was paying). Drivers were again on water and soft drinks and everyone else stuck to soft drinks. Nephew ordered a Korean cocktail to start. Later orders a bottle of Soju. His drinks bill alone was £20. He's on a carb diet and orders an extra plate of Bulgogi for himself, and shared the rest of our BBQ.
The lad seems to know no limits. I've told him previously that whenever you go out with friends (or in the future at work), always be aware of what others are ordering so you do not stand out.
This lad lacks any type of etiquette. Never says thanks and never says hello to elders. Do I just suck it up and ignore it.