I look at it more as establishing boundaries with people so that situations like these are avoided. It seems (although its impossible for me to know) that you've made everything clear about what happened and your sister 1) is still with this guy and 2) isnt sympathetic to you. If I was in your position the boundary would be just not letting her into my life anymore. This is what I've had to do with my mother who has been very abusive physically emotionally and in a weird sexual way when I was younger. You have to only allow people into your world as much as to where they won't hurt you anymore (if that makes any sense, i dont think im saying it right). Maybe just deal with your sister cordially and superficially at family functions for a while. Focus on yourself.

To simplify it: a boundary is with the entire relationship, to prevent problematic situations.

I hope i'm making sense, my mind is mush today.