Vaporisas, I've read this entire thread and all the advice that has been given to you and I have to agree with flamingojohn and to an extent with AZB.
IMO, there are some things that you must do, regardless of how difficult they may be for you:
1. You have to disconnect yourself from the situation and stay out of it. There's an old Dominican adage that says that nobody should interfere in problems between a man and his wife, and in this case you should follow it. As AZB pointed out, you do not know every single detail of your parents' relationship, and as such you have a slanted view of the situation. Your side of this story may not necessarily reflect the views of your mother and/or father.
Infidelity, divorce, arguments between a man and his wife, etc., are all a part of life. Unfortunately for some people, it is a big part of their life. In your case, just like in most, the fault can probably be attributed in part to both your mother and father. Don't try to figure out who's more at fault, and be confident in the fact that it most certainly is not yours.
2. You have to stay level headed and keep your cool. As much as you may want to yell at your father or tell your mother, it is not going to resolve anything, and will most likely worsen the situation. You must realize that life goes on, and that torturing yourself over this will only cause you to loose sleep, appetite, happiness, and valuable time. As mad as you may feel, loosing your cool is never good.
3. You have to find an outlet for your feelings. You mentioned that you have a sister that lives with you, and somewhere else you mentioned a brother. We don't know their ages, but if they're somewhere around your age, they are probably aware of the same things that you are. Spend time with them and realize that they probably need as much support from you, as you need from them at this time. Whatever you do, do not neglect your school work. If anything, use that as a way to occupy more of your time.
I know that you are going through a very difficult time in your life, and that the range of emotions that you're feeling is tremendous. Take it from some of us who are a little older, and who have gone through different tough times of our own; you will make it through this! Stay strong and best of luck!