I'm sorry for the loss of your father
. As far as your brother, have you talked to your mom about getting some help for your brother. Greiving comes in all different time frames and different ways of handling things. Age is a big factor and probably sex could be too. Your brother was 13 at the time and could have been when he needed your dad the most. Growing up is hard enough. If he is into drugs and alchohol, it sounds more like he is trying to numb his pain more than an acceptence issue. It probably would be best to find a place where he could get the right kind of help. Not just for the drugs and alchohol, but also for his loss of a parent.
If he doesn't get the help soon, he could spiral downward and things will only get worse.
You mentioned that you are doing pretty well. That is wonderful. Have you ever talked to your brother recenty about how he is feeling? Sometimes people just need to talk it out. Keeping it bottled up inside is not healthy. What better person to talk to would be your sibling. I'm sorry to hear that it is taking a tole on your mom. It has to be hard for her to deal with the loss of her Husband, let alone trying to deal with a son who is going through a crisis.
My best to you and your family!