What do I do?

I am so confused on what to think.
My best friend, we are closer than ever.
And now, she is trying to get with my ex-boyfriend?
Knowing on how much stress and pain he put me through.
And she told me to break up with him?
Now I see why.
I just, don't know what to think.
He hurt me, and I don't want him to hurt her like he did me.
She won't listen to me when I tell her.
What do I do?
Just watch her get hurt?
Watch her go down?
Then say what?
"I told you so."?
She keeps blowing me off when I try to call her and talk to her, not even about him.
I have only brought him up twice. That's IT.
And, I am completely confused.
We have been through EVERYTHING together. Literally, everything.
Our periods, first loves, first fights, first year of junior high, first year of highschool, first depressions,
love for our first phone, everything.
And I don't want to loose my best friend, better yet, my sister.
What can I do?