I think the problem is actually that they are all still quite small. (the cousins I am talking about are aged 5-12)
Kids don't share properly until age 3 and then it is a process to be learned, so I think that might be a key.
I think in your situation I would say it to them even in front of parents, in a subtle way! So if older one was saying poop head or whatever I would say 'that isn't a kind name to use, can you think of a nice one? How about gorgeous chops? Or eagle eyes? Can you give yourself a new superhero name?' etc
Or if they are mimicking, laugh at them gently - you sound like a parrot, they are always copying things. Can you do any other animal noises?
If one of your dcs is upset, then maybe crouch down at their level, your arm round your dc and say 'dc doesn't like it when you do x. It makes him upset and then he doesn't want to play. Can you be kind and play nicely?' When it is against younger ones, say collectively are you big boys being kind big brothers to the little ones? Oh great I knew you knew how to be kind big brothers.
That sort of thing, not 'telling off' but redirecting in a positive way.
best I can do I am afraid, a lot will depend if other mum minds, or likes it and picks up in it, or has a wobbly. She may not notice it any more, because she has given up on it.