Right....Im gonna fix this one easy for you!

Go up to him a say the following words....

"insert name of your brother" I have been smoking weed for a while now and thought I would get it off my chest and tell you because I dont feel comfortable lying to you about something so silly.

There we go your worries are sorted.....just think whats the worst he could do???? Are you going to be hung-drawn and quarted for telling him?????? I hope not!