Divorce is not the end of the world... I know. My parents divorced when I was 16. It sucks, but there is nothing you can do about it.
Listen... I know this is not what you want to hear but marriage is not a life contract. It should be, in a perfect world, but it isn't. People get married and unmarried every single day. They fall out of love for many reasons. It has nothing to do with you.
You have to see your parents not only as your mom and dad, but as individual people too. People with needs, feelings and dreams. When my parents realized that their marriage was dead, they made a huge effort to try living as a normal family. It was horrible. Where I used to see them joke around and hug in the past, they now sat in a room, each in their own world, polite but with no emotion. It wears them down and it's not healthy for you or your little sister either. I know you find this hard to believe, but you will see things differently when you grow older.
You're 15 years old. believe me when I tell you that you have a lot of strength inside you that you are not even aware of. You need to reach inside you and find that strength now. You need to talk to your little sister. If you are confused at the age you are, can you imagine how she must be feeling? She needs you now. So does your mom and your dad.
You all need to be strong to work through this and you will. You're probably afraid of having to take sides between your parents. You don't have to. They will always be your mom and dad.
You MUST FIND SOMEONE TO TALK TO. Your friends will want to help but I'm afraid that being as young as they are, they might not give you the best advise. You need someone older that can really listen to your concerns.
I found a very good friend in our school counselor. She did not judge me. She listened to my fears which is what you need now. She also gave me some very good advise. Try and speak with your counselor.