Wow, I'm so sorry to hear that. I had a friend in college take his own life. Both us (his friends) and his family looked very hard for a "why" or any signs that were missed. He was by all accounts a happy and outgoing person. He left no notes or communication to anyone, nor did he have any obvious motive to do so. However, going through is personal effects we discovered he had been planning it for some time, since he had bought several items that could only serve one purpose...He had apparently attempted several times in the days preceding and had hidden it from everyone, until one night he said he was going out for a walk and jumped off the parking garage 2 blocks away a few minutes later.
It's often said that people who actually do go through with taking their own lives could never have been stopped, and those who are "saved" are seeking help from others and never intended to carry it to completion in the first place. In the end, I view serious depression no differently than any other illness, and I'm terribly sorry that it struck your family.