You have my heartfelt sympathies...and those, I suspect, of thousands of other families, who wonder "what went wrong?" when a sibling goes--as they say--sideways, before plummeting downward.  

Same home, (often) same parents, same time in history, same schools, same food, even.  Doesn't matter.  Those around him or her make it, one way or the other, and are reasonably happy, but one...just...doesn't, often without even the momentary success Peter enjoyed.

The young man I consider the brightest, most talented individual in my BCIT broadcasting class in the late 1960s, was, less than a year after graduation, wearing a hardhat and steel-toed boots as a member of a highway paving crew.  A short few years later, he was dead...reportedly by suicide.  

What happened?