I'm going to tell my brother he'll have no job prospects if he majors in psychology
Why would you say that to your brother?
He is my brother, and I need to look out for his best interests.
I understand wanting to look out for your brother, but while the field of psychology might not be a good fit for you, it might be something where he will excel. Rather than discourage him, I think you should support his choice.
He's my brother but he's also my nemesis. I can't worry about something as small as that.
It's natural for siblings to have a relationship that has its ups and down, but ultimately I think you should building him up, rather than tear him down. It's what you would want him to do if the situation was reversed.