That's very sad... about your brother and your friend.
You're right , there aren't always the classic tell tale signs that someone will do this. I've read where, even when they have been very depressed of late, once the decision has been made to end their life, they can appear bright and bubbly so people around them breathe a sigh of relief... but the reason fort the brighter outlook can be because they know it will soon be over for them. Even people trained in this field can't always pick it, as everyone is different.
I also think that it's not a reflection on how much you do or don't love your family, because once you get to feeling this down then you're no longer who you really are. You've already sunk so far down that reality isn't working. People can keep telling you things will be ok, give it time, but in this state of mind all you can see is doom.
I've never agreed with the statement that committing suicide is a cowards way out. I hate when people say that. To me, it just means that someone has gone to the very depths of sadness and their take on reality is far removed from what's real. It can happen to very normal, everyday people who seem to have it all.
I feel I have to add, that having said all of the above, I do not believe for one second that Allison committed suicide. Just thought I'd add that incase it's misunderstood.