What to do, my dad has depression?

Over the past couple of years, my dad has been heading in a downwards spiral. It all started, really with the death of his brother, he was extremely close to him and it was a huge loss not just for him but for me. My dad has terrible back pain due to an accident he had over 10 years ago, which caused him to slip a disk in his back however it never got sorted and he has arthritis at the base of his spine, he is in constant pain and is heavily medicated. It's been a month or two since the drinking started, and its not just a beer or two, it can be as many as 4 bottles of wine a night. He then breaks down in tears and gets angry. It's an awful state to see, not only for me but for my 10 year old brother, who doesn't have a clue at what's happening, i realise this isn't very descriptive but witnessing it is much more of an impact. My family, is pretty much dis functional, if i'm being honest, my mother and father have never gotten along, and my mother isn't the most compassionate person ever. She seems to think it will just fade away but i highly doubt it. My dad has also been saying some quite terrifying things about ending his life etc. I headed downstairs just earlier on, and he was in tears talking about his first wife, who died and who he said was his true love and how much he hates life now, with my mum. Its just hard to be okay with your dad saying that when your mum is upstairs. I just don't know what to do, its taking its toll on its family, and is personally having a negative effect on myself. For the past couple of weeks, my attitude at school has changed, i'm not showing any interest any more, and i tend to cry myself to sleep, i don't want to come across like i want a sympathy vote as i'm really not the attention seeking type, i just need some advice on what to do.