At some point people need to stand up for themselves.
I'm pretty wimpy person myself.
But at some point, it has to be about me.
Your sister made her choices in life. She has to figure that out, its not up to you.
The stuff that belongs to your dad, well that is up to him.
The stuff that belongs to you, that is yours and you have control of it.
Rewarding a bully by letting him bully more probably is no answer to the question.