It's a really good thing your W was able to open up to you about her brother. I'm not sure if it's something to do with being British (you know the who stiff upper lip, do it for mother England thing) but my H has also bottled in his feelings about his dad's death two years ago. It was then that his first A started.
I, like you, tend to try to fix things. It's our attempt to maintain some sort of order and control over our world. And, for me anyway, I'm a control freak because I have a very hard time trusting people. For you to start to change the dynamic with your W is important.
I know there has been much discussion regarding your W's drinking. My H went out drinking every night to avoid coming home and dealing with his real life where his W was depressed and his dad had recently died. Your W seems to show many of the same behaviors.
I wouldn't waste too much time worrying about her drinking. Instead I would try my best to encourage her to do things to make herself happy, to put herself first (even before your kids) without feeling guilty, to get a job if she wants one, and to spend more time with her friends. So long as she is depressed you are never going to be able to fix your M.
