I told my ex's brother he had a drinking problem and had threatened suicide--that's what ended our relationship. He told his brother a bunch of lies about me, and the classic alcoholic tale that he only drank because of me. His brother believed it and off they went together into dysfunctionland, leaving me feeling like a complete fool.
His brother told me both of their parents were alcoholics, and that often times when he talked to his brother on the phone my ex was slurring his words and acting overly emotional. Still he chose to believe my ex's lies. You can't assume the family member you choose to tell is going to be healthy and not prone to denial. I've told my mom a hundred times my brother is a drug addict. She refuses to believe it.
I agree with everyone else: stay out of it. In my case, telling on my ex was just another act of codependency. I couldn't save him myself so I was trying to enlist the help of outside agents.