What if I tell my brother and his wife they can't afford to have a baby?
Why do you think that's a good idea?
Because if they had a baby, it will cost too much to take care of her
It is wrong of you to even consider telling your brother and his wife not to have a baby. You need to mind your own business and stop imposing your opinion onto others.
I don't think it's the right decision for them.
I think you're crossing boundaries here. They're adults capable of making their own decisions. While I'm sure you mean well, I doubt they'll appreciate your recommendation on how to live their lives.