Hi,
I've been on anti-depressants on and off for 6ish years, and also when I was 11/12 I was on Prozac for a brief time. This is just a rundown of how I experienced many anti-depressants and other psych drugs in my life. I've been on a LOT of stuff, and a lot of it didn't do shit, but these are the only ones that I've had a change in suicidal thoughts/behavior on. I've probably been on like 12-15 psych meds, and the majority did not have bad side effects. MOST people do not have increased suicidal thoughts from anti-depressants. It definitely happens, but it is not that likely IMO. And note as I explain these things, I wasn't in any more danger of hurting myself, just my thoughts were troubling, and I can (usually) control myself enough to get past rough spots like that, and of course I told my doctor right away. Although, the exception, I think Prozac really made me more dangerous to myself, but I only took it for maybe 6 weeks because I did not like how it made me feel.
Prozac made me kind of almost manic (I'm not bipolar), and just kind of consumed with anger, suicidal ideation and even some homicidal thoughts. I stopped taking it without anyone knowing for a while, and then somehow or another they took me off it anyway. Remember I was a pre-teen so I had little say in what was happening. Even then I was kind of pissed that my parents were trying to "fix" me with a pill, but regardless, I think Prozac made me more prone to anger and perhaps violence against myself or others if I had kept on taking it. And this is coming from a very passive and certainly non-violent child. Then again I was young and perhaps my perception is a bit warped. I think pre-teens should not be administered SSRIs, but that's another topic altogether.
I was on venlafaxine recently, and I did have an increase in suicidal thoughts, but I believe it was because Cymbalta just worked better for me, and since I've been back on Cymbalta I'm okay. But I'm also 25 now, so not technically a "young adult", in medical terms.
A mood stabilizer, Lamictal, also increased my suicidal thoughts in a different way. I was so apathetic and dissociated/out of it before I started taking it, that once it actually helped me feel a little better, it was easier to think and rationalize killing myself. I also would get these kinds of vision things... like when I went outside to smoke a cigarette, I'd just kind of see myself jumping over the railing (I live on the 4th floor of my building). I didn't really have a thought accompanying it so to speak, but still that was new to me. And like I said I was actually feeling better, but probably technically having worse suicidal thoughts. I still get this sometimes, and I'm still on Lamictal, but it's gotten a lot better and I don't get anxious as hell when I get those kind of vision-things.
I hope this makes sense and perhaps you can look out for any signs of stuff like this with your brother. He needs to report any kind of suicidal thoughts/behaviors/impulses to his doctor right as they are happening, if they happen. Regardless, the majority of people seem to painlessly adjust to SSRIs/SNRIs.
I agree with the above though, treat him the same and be a good to him. You sound like you care a lot about him. Hope all goes well.