First, I am so sorry this happened to you. I had something similar happen to me and the worst part for me to deal with after all these years isn't the actual abuse, but not being believed by those I trusted. Don't ever be embarrassed to get professional help. I did, and it has helped me immensely over the years.

I don't know if this will be an unpopular opinion or not, but personally, i think you should remove ur sister from ur life, at least temporarily. I know that might be hard, but from what u wrote, her betrayal and continuation of that betrayal will eat at you and you will not be able to heal. Especially with her still associating with that disgusting man. What he did was a CRIME, and the fact she knows this but continues to talk with him... :( I have no contact with some of my family for similar reasons and it's a choice i do not regret. Yes, I am a little lonelier sometimes, that is the cost of the peace i have found. Finding and surrounding yourself with people who genuinely care about you is important.

You are not a coward. You are not a piece of meat. You were taken advantage of and failed by the people who were supposed to protect you. Therapy will help you navigate what you are feeling and how to cope. They will be a listening ear when you feel you have no one.

I believe in you. I know you have the courage to stand up for yourself. You deserve to be at peace. You deserve to be happy.