I'm going to tell my best friend about his brother's shit talking
Why do you want to ruin their relationship with one another?
Cowardly people like him deserve what they get
I think that you should let them work out their issues on their own. It is not okay to try to interfere in other's relationships.
They are both kids and they can't be trusted to work out their own issues.
You should stay out of other people relationships. If they are kids they need to go through life's ups and downs and work things out in their own time. That way they will have skills to deal with life later on.