What to do, what to do?

I am really fucking confused. One of my very best friends is involved in a really unhealthy and emotionally abusive relationship. I can hardly stand what it makes her do and say. She throws away her own worth and stands up for her right to fuck herself over. If she wasn't someone I love deeply I could probably just shrug and walk away, but I really don't know what to do.

The person she has this icky relationship with is her sister, and right now she lives with her sister and her sister's fiance. Every time the 'happy' couple have an argument, my friend jumps right in and starts freaking on the guy to 'protect her sister'. I have tried to tell her that it's not her place, that it is only hurting all of them, but she just goes on defending her stance. Her goddamned sister is more then happy to let my friend screw herself for her benefit. I don't know what to do, I don't know how to let it go, or how to make a difference. I fucking hate this blind bullshit, when people tell themselves lies to justify bad behavior. Cowardice and dishonesty disgust me and the last thing I want is to feel this way about someone I love, especially since I know she is better then that.