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I believe that I can help you.
What you need at this point is closure. You cannot make your sister act like a kind and caring adult, and perhaps you cannot get closure with her anymore.
I recommend a 2 stage approach.
First sit down and slowly compose a letter to her. Take your time. Do it over days or even longer. You can do it on the keyboard but ultimately you will want to write it by hand and sent it with an old favorite photo of you and her together or one of your entire family together.
Tell her that you just want to get closure for yourself from the hurt you have felt from someone you thought who would be there throughout your life. Tell her that you are sorry for the loss, but accecpt it, and that no matter what happens, you are her sister and will always love her.
You may not feel that way at the moment, and may never again, but you shoulld be positive.
Tell her farewell and that perhaps you will meet on the other side. WIsh her a good life, and when you are satsified that the letter is as good as you can get it, then write it by hand on nice stationery and send it to her, perhaps with a whiff of some scent on it, just to give it more presence.
Then forget about it.
In the meantime, I urge you to purchase the following workbook on thought-stopping, and go through the exercises in order to strengthen youself and steel yourself from these sad thoughts.
Perhaps your sister will grow up one day and ask for your forgiveness. In the meanwhile, do the best to keep her out of your thoughts, and not live on hopes that may never occur.
I wish you great healing and shall keep you in my prayers.
Elliott, MAE, LPCC, NCC, CCMHC