Can I tell my brother to wash his hands?
What is causing you to tell him this?
I have seen him putting his hands in his underwear after putting on his clothes.
It makes sense that you would ask him to wash his hands after seeing him put his hands in his underwear. At this point i guess you are just looking out for his health. Good that you care about his well being!
It's more like I really want to torture him. I am going to get him to wash his hands just because he is my brother.
You say that it is to torture him, but washing your hands isn't really a form of torture at all? I think that you just care, and caring is good. He should take more care of himself and it is good as a brother that you're looking out for him.