I was misdiagnosed with first major depression, and then bipolar disorder in high school (I really just have ADHD - funny how not being able to do well in school or interact effectively also makes you sad).
I can tell you that standard anti-depression meds like Zoloft, usually are not concerning. I know several people who have been paramedics or in the health care field for years who take them, and I doubt they have any significant impact on their ability to effectively provide care.
I don't believe anti-depressants are typically used for manic depression though.... mood stabilizers are. I was put on an anti-psychotic, and from personal experience I can tell you there's no way I would want to be driving or providing care while on that drug (Quetiapine). That might just be me though... and I didn't even have the disorder they were trying to treat.
Interesting also, that years after they got my diagnosis right, I was given the same drug, in a LOWER dose, as a sleep aid. Pharmacology is weird.
It's really hard to say without knowing the kid or what drugs he's on. The other thing is, some people with severe manic depression that isn't well-controlled may not be the best caregivers... they can be very impulsive and make poor decisions while in manic episodes.
Suggesting that the brother have an honest discussion with his psychiatrist might be a start. Hopefully he is responsible enough to be concerned about whether or not he should get in to this field rather than just whether or not he can get away with it. I know that the Board of Nursing sends nurses and CNAs that they have learned have some kind of mental illness to an independent psychiatrist to evaluate whether or not they are able to work in such positions. Those who are deemed unfit can lose their certification or license. I don't know how they get to the point of sending someone to a shrink for evaluation in the first place, and I doubt that most EMS certifying bodies do this, but interesting nonetheless.