I'm going to tell my parents that my brother is buying and smoking weed
Why do you feel the need to rat on your brother?
Because I can't keep it a secret anymore, and I also want to get back at him for something he did.
I think your intentions may be a little suspect, but the act itself is good. Parents should be told when their kids are doing drugs, since drugs are very harmful.
Well my brother has been doing this for a long time and I've been feeling bad because I've had to keep it a secret from my parents.
You seem to care about your brother and definitely should let your parents know about his drug habit. It's necessary to stop him before it's too late.