How do I tell my parents I think my brother has psychiatric disorder?

Parents. This is hard but in some way they may already supect it. It is best to gently discuss this with your brother and see what he says, then gently ask you parents and see if their reaction. Ask questions like what if we told mom and dad? Or what if someone we know had a serious illness? Move from the general to the specific. It may be best to consult and pay for a counselor just for this issue.
Psych illness. Its best to encourage your brother to get a psych eval and let him decide what information he feels comfortable letting anyone know unless he is danger to self, others or unable to care for self then you have to call 911 or take him to nearest er if possible.