Have you asked your mother why? There can be all sorts of reasons, they'd be different for each individual; the way things look to you might not be how they are or look for your mother, that's always the case. We can't really know another completely, especially is it's a parent (or child) because of the differences in age. But like Junerain says, your mother is very lucky to have you, in there trying to understand and help. But we can't really help another or live their life for them and sometimes what they do affects us well/badly which is unfortunate.

It could just be a matter of a bit more time that your Mom needs or she may know something about your father and the marriage or herself that you don't. I'm sorry it is so rough on you, being in the "bottom" postiion. I know about that a little, I'm the youngest of 4 siblings and have always hated how I couldn't quite understand or see things from their perspective because I was so young at the time. It's not You or anything you're doing or not understanding, it's just that you can't do anything about your "time" or perspective. You're at the bottom of the tree, on the forest floor, and they're all battling it out up in the treetops.