Be preachy
"The world will not be destroyed by those who do evil, but by those who watch them without doing anything."
I avoided this for a long time because I didn't want to be that annoying guy always preaching to others. But I don't care now. The people who are going to listen will be helped, while the others will just ignore it. At worst, they will make fun of me.
If the people I care about are making a mistake in front of me, I am going to voice my opinion. That is what I expect from them as well. Whether they follow through or not is not my concern since I can only control my actions, i.e., preaching. I can increase the success rate by conveying it in the right tone, but at the end of the day, you can't help someone who doesn't want to help themselves.
Regardless, it's your duty to let your bro know that he's going down the wrong path when he does. Don't be fooled by fake niceties and supposed politeness, and let your friend make a mistake that he'll regret for sure.
Also, keep in mind you shouldn't be too controlling. Freedom to express your views doesn't give you the right to infringe on others' freedom.
If they can't learn from others, let them learn through experience. Don't abandon them just because they didn't take your advice.