From an older similar thread, teach him to say, “I’m being repressed” to anyone nearby any time he is gets scolded or asked politely to refrain from whatever he is doing.
"This is my rifle this is my gun…
…This is for fighting this is for fun!"
Along with appropriate gestures and possibly a toy gun, if no real M14’s are available.
This is just EVUL.
If you did this to my kids, I would have pulled out your short hairs.
However, because society is what it is, they learned this HORRIBLE phrase all on their own and I now have to resort to " I’m not listening…lalalalalala" when DUUUUUUUUUDE shows up. The DUDE does not abide.
BOT,
Whatever you teach the evilspawn of your siblings will come back tenfold when you have spawn of your own.
Hit submit too early.
I have taught the spawn of my Born Again Evangelican Christian SIL The Shocker.
Subversive and so very wrong for the Whitest of the White Bread Family you could ever meet.
You are welcome.
Lots of funny and intelligent ways to treat a kid here but I think you worked it out in the OP:
But have the courage of your convictions and trick the poor little kid into saying, “My mummy is a big fucking slut.” Laughs all round.
When we were kids, we called that game “Look!” Tap someone on the shoulder and say “Look!” and then open your mouth when it’s full of some really gross-looking chewed food. Extra points if some food falls out.
I helped teach a friend’s nephew the word “Dude” as one of his very first words. Well, one of the kid’s other aunts is now known as Aunt Dude, and it’s still sticking although the kid’s 10 now. Ha!!
When my sister was a kid, her 80-year-old babysitter and her bridge buddies taught my sister to pretend to play cards with them while “smoking” their cigarette butts and taking a big gulp of Listerine (the old-school yellow kind that resembles beer) when my parents came home. Apparently to much outrage followed by bemusement. Who knew a bunch of elderly hens could be so devious…
Just sayin’. It’s a thought.
My cousin was very, very upset when her white trash in-laws taught her son that he could pee outside (off the deck was most popular).
You beat me on both of these. There is very little in this world as annoying as a young child who has just learned to do armpit farts.
Loud obnoxious toys are good. I have a 3 year old nephew too and so far have bought him drums, a guitar and a keyboard. Even at six months he had a very loud rattle to wave about. Why yes, my brother did torture me as a child…
Can you still get those kids’ sneakers that light up and squeak with every step the wearer takes? They are so fucking annoying! They’d drive the bro and sis in law mental…
Also, don’t know how conservative your bro and SIL are, but I love when my nephew notices and is fascinated by my tattoos or piercing. ‘Will I take you to the piercing parlour when you’re 16?’ I coo at him in the full hearing of my brother. ‘Yes I will! Yes I will!’ I also did this when painting my nails and my nephew comes up to me wanting some polish on his nails too. My brother is kind of homophobic and he went nuts about that one. Telling your three year old nephew how much fun Gay Pride is, and that you’ll take him the next year is freakin hilarious.
I wouldn’t do anything that labels the SIL as promiscuous, though; that’s a bit unfair. Nothing wrong with having a healthy sex life in your past. 
There’s a long back story that makes it cheeky fun instead of cruel fun. I’m a smartass not a jackass.
Teach the child to sing the fight song of the bitter rival of Daddy’s favorite team.
I met my boyfriend’s extended family over Christmas and had a conversation with his uncle Tommy, who taught my boyfriend to walk up to women and say, “You smell like a french whore” (though he apparently messed it up and told women the smelled like french horns) and he taught one of the cousins to respond to the question, “What are you doing?” with the phrase, “I’m looking for big tits!” You could probably use either of those to wreak some havoc.
buy him a big bag of little kazoo’s, then make sure to stash them in every possible place in his luggage and anywhere else you can find to make sure some make it home with him.
There’s nothing more fun than teaching toddlers to be crafty. I highly recommend that activity be used with found objects. Things like tampons (“Look mama, I made you a necklace!”) or condoms are usually best. And trust me, I speak from experience as a precocious 3 year old myself.
Also, along with the commando thing, perhaps illustrate that only the Grown Ups do the special underwear wearing. You know the kind, that requires such pieces to be worn outside / on top of the clothes. Illustrate yourself for days on end and you’ll have a convert for sure.
In that case, train the nephew to hug the knees of every male he meets and say ‘Daddy?’. Particularly effective with priests and general society pillars. Or - oooh - even better - train him to hug the knees of women he meets and say ‘Are you my mummy?’. If he says it like the creepy Dr Who WW2 kids of last year all the better.
i.e., Behold, the Power of Dude.
**Alienhand **- we look forward to a full report on your handy work.
The zombie march.
Hands in front, limp at the wrists, staggering about while growling for “BRRAAAAAAAAIIIIIINNSSSSS”!
I taught a friend’s niece this one and the mileage on it is awesome. it was especially great when she put on a McCain halloween mask and moaned around the store as a midget “zombie mccain”.
To the tune of Turkey in the Straw:
The old crow flew into the county store
and he make raspberry sound on the counter
and he raspberry sound on the floor
and he raspberry sound in the coffee
and he raspberry sound in the tea
and if I hadn’t moved he’da raspberry sound on me!
Best done with crackers in the mouth.
I taught that one to my grand-nieces over a day when I babysat. My nephew wouldn’t speak to me for a month.