My advice?

Stay out of it. Don't get involved in other peoples problems. If they ask for your help, give it, but otherwise, don't do it. Stand back and observe, be peaceful and helpful, but don't interpose your own ideas into a situation that doesn't ask you to be a part of it.

Your sisters grandson...

Do you even see this kid at the family reunions? If so, you can skip if he's too bad to be around, but otherwise...

Don't ask for trouble.

For every "bad kid" that has trouble when they are five, there are dozens that outgrow them.

Bad diet is the cause? Maybe. Or possibly he's simply high strung, or resistant to being pushed around? The point is, unless you live with him and have the ability to understand and empathize with others, you probably wouldn't be able to tell the difference.

Beating the kid into submission (the popular choice on GLP!) only works on certain people. On others it's the kind of thing that causes the problems in the first place. each child has to be handled responsibly, and by the people that know them best.

In the end, it isn't your job and trying to do it will only lead to grief for yourself and others.