I'm thinking of criticizing my nephew even though he's my landlord
Why do you want to criticize your nephew?
I'm trying to help him improve his life.
Giving your nephew constructive criticism is okay and can help him to improve in necessary areas. If you don't point out the issues that he needs to fix, he will likely be unaware of them and make unnecessary mistakes.
That's right. He's kind of dumb and always makes silly mistakes.
It's fine to give someone constructive criticism. But since he's your landlord, you have to be aware that if he becomes offended at something you say, you might be without a place to live.