It's no longer your job. It never was in the first place.
If you were still involved with your ex, this would be an appropriate and supportive conversation to have with his brother, but as you are not, they are now trying to drag you back into drama that is no longer yours.
Just as you couldn't help him quit drinking, you can't help his family learn how to deal with the drinking, and what is more, if you provide all the information for the intervention then you are setting yourself up for potential blame from them when it doesn't work. You don't have all the answers, and you don't
have to have all the answers.
Let him go.
