Can not handle sudden death of my sister

My sister and I were a unique set. My mom had us both out of wedlock and we were the only two children out of seven that had her maiden name. When I was about fourteen months she and I were put into a hospital because of neglect and then into a foster home. We were eventually returned. When I was seven I was taken away again for multiple types of abuse, never to return home. My sister pattie and I kept connnected as much as possible and I did get to visit her some as a teenager. I always felt guilty for not being there because I knew she was probably suffering instead. As adults we were like twins always knowing what the other was feeling and going through. I was always very protective of her and talked and messaged her on a regular basis.
March 2, 2018 the day before my wedding anniversery, my sister unexpectedly died. I had just talked to her two days before.
I haven't been able to process this at all. I took her pictures down and avoid anything that will remind me of her. Since she died all my other siblings but one ignore me, I think it is because we looked so much alike and so different from them, that they don't want the reminder.
I am terrified to try accepting and processing this. The only other significant death was my son of two weeks many years ago, and I ended up suicidal in state hospital for five weeks.
I am raising three grandchildren and am afraid to give in to my loss in any way, but I m definately stuck in place and lost and not sure how or what to do. I have terrible nightmares when i do sleep, and don't want to go back to sleep after.
I do not know where to begin.