Andrew, my heart goes out to you. I can't offer much, but I can say please don't give up.
The first advice we hear when we come here serves me well: take deep, slow breaths, look around and place yourself in the room. Focus outward, take a walk. It is good that you reached out to your therapist.
Remember there are people in your life, as well as here at MS, who care for you, and even love you for sharing your pain. You've helped me a number of times in the past, and I'm grateful and lucky.
If your belief that your grandfather never got to your sisters got you through your horrible experiences, then maybe it did its job.
In other words, you mustn't blame yourself - if that is what I am hearing.
Maybe you'll be able to reach out more to your sister? Certainly her understanding of your situation is deeper now, and it is possible she feels similar to you.
I wish I could give you a hug, (((((((Andrew))))))))