Okay so I screwed up my relationship with my best friend (for 6 years) (I'm 13) and I did that by doing "stuff" with her brother. In her bed. I know I'm a terrible person but I had to be honest and tell her. Now she just calls me a slut and says she will never speak to either of us again. I need help because I can't afford to loose either of them.

Only time and proving that you aren't that 'horrible person' that can't control themselves might fix this.

You did "stuff" at 13? And you wonder at being called a slut?

The only thing you can do is throw yourself on her mercy. Tell her you were very stupid and realize what a dumb mistake you made. Ask her help in recovering from this mistake.

As for her brother, he took advantage of you and you are better off without him.