I agree with what other posters have said. These are their life choices, not yours.

I also think you should reach out to his school guidance counselor. He likely is suffering in school because of what is going on in your home. He is probably worried all the time.

Your parents likely would not permit him to go to therapy but they cannot interfere with school policies on guidance if your brother is failing. The school should know how hard things are for him right now.

You can't parent him but you can be an adult who is kind, who he can trust and who he can depend on. Anything he is interested in, try to support. Let him know you see what he sees in your home. Let him know he is not alone.

Take him out for an overnight or weekend to someone else's house if at all possible. Offer to drive him and his friends to places. Anything to help him connect with life outside the house.

The two of you should also try Al-Anon or Alateen meetings. They are for family members of alcoholics. They are free and meet 7 days a week.

Good luck.